Chapter A General rules of electrical installation design
Chapter A General rules of electrical installation design
Chapter A General rules of electrical installation design
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2008<br />
http://theguide.schneider-electric.com<br />
Technical Series<br />
Electrical<br />
<strong>installation</strong> guide<br />
According to IEC International Standards
This technical guide is the result <strong>of</strong><br />
a collective effort.<br />
Technical advisor:<br />
Serge Volut - Jacques Schonek<br />
Illustrations and production:<br />
AXESS - Valence -France<br />
Printing:<br />
Les Deux-Ponts - France<br />
Edition<br />
March 2008<br />
Price: 90 �<br />
ISBN: 978.2.9531643.0.5<br />
N° dépôt légal: 1er semestre 2008<br />
© Schneider Electric<br />
All rights reserved in all countries<br />
This guide has been written for <strong>electrical</strong> Engineers who have to<br />
<strong>design</strong>, realize, inspect or maintain <strong>electrical</strong> <strong>installation</strong>s in compliance<br />
with international Standards <strong>of</strong> the International Electrotechnical<br />
Commission (IEC).<br />
“Which technical solution will guarantee that all relevant safety <strong>rules</strong> are<br />
met?” This question has been a permanent guideline for the elaboration <strong>of</strong><br />
this document.<br />
An international Standard such as the IEC 60364 “Electrical Installation<br />
in Buldings” specifies extensively the <strong>rules</strong> to comply with to ensure<br />
safety and predicted operational characteristics for all types <strong>of</strong> <strong>electrical</strong><br />
<strong>installation</strong>s. As the Standard must be extensive, and has to be applicable<br />
to all types <strong>of</strong> products and the technical solutions in use worldwide, the<br />
text <strong>of</strong> the IEC <strong>rules</strong> is complex, and not presented in a ready-to-use order.<br />
The Standard cannot therefore be considered as a working handbook, but<br />
only as a reference document.<br />
The aim <strong>of</strong> the present guide is to provide a clear, practical and stepby-step<br />
explanation for the complete study <strong>of</strong> an <strong>electrical</strong> <strong>installation</strong>,<br />
according to IEC 60364 and other relevant IEC Standards. Therefore, the<br />
first chapter (B) presents the methodology to be used, and each chapter<br />
deals with one out <strong>of</strong> the eight steps <strong>of</strong> the study. The two last chapter are<br />
devoted to particular supply sources, loads and locations, and appendix<br />
provides additional information. Special attention must be paid to the<br />
EMC appendix, which is based on the broad and practical experience on<br />
electromagnetic compatibility problems.<br />
We all hope that you, the user, will find this handbook genuinely helpful.<br />
Schneider Electric S.A.<br />
The Electrical Installation Guide is a single document covering the<br />
techniques, regulations and standards related to <strong>electrical</strong> <strong>installation</strong>s.<br />
It is intended for <strong>electrical</strong> pr<strong>of</strong>essionals in companies, <strong>design</strong> <strong>of</strong>fices,<br />
inspection organisations, etc.<br />
This Technical Guide is aimed at pr<strong>of</strong>essional users and is only intended<br />
to provide them guidelines for the definition <strong>of</strong> an industrial, tertiary or<br />
domestic <strong>electrical</strong> <strong>installation</strong>. Information and guidelines contained in this<br />
Guide are provided AS IS. Schneider Electric makes no warranty <strong>of</strong> any<br />
kind, whether express or implied, such as but not limited to the warranties<br />
<strong>of</strong> merchantability and fitness for a particular purpose, nor assumes any<br />
legal liability or responsibility for the accuracy, completeness, or usefulness<br />
<strong>of</strong> any information, apparatus, product, or process disclosed in this Guide,<br />
nor represents that its use would not infringe privately owned rights.<br />
The purpose <strong>of</strong> this guide is to facilitate the implementation <strong>of</strong> International<br />
<strong>installation</strong> standards for <strong>design</strong>ers & contractors, but in all cases the<br />
original text <strong>of</strong> International or local standards in force shall prevail.<br />
This new edition has been published to take into account changes in<br />
techniques, standards and regulations, in particular <strong>electrical</strong> <strong>installation</strong><br />
standard IEC 60364.<br />
We thank all the readers <strong>of</strong> the previous edition <strong>of</strong> this guide for their<br />
comments that have helped improve the current edition.<br />
We also thank the many people and organisations, to numerous to name<br />
here, who have contributed in one way or another to the preparation <strong>of</strong> this<br />
guide.
Guiding tools for more efficiency in <strong>electrical</strong><br />
distribution <strong>design</strong><br />
Technical knowledge Pre-<strong>design</strong> help for budget<br />
approach<br />
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Articles giving base skills about<br />
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Techniques”<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Selection criterias & method<br />
to follow in order to pre-define<br />
project specification:<br />
b Architecture guide<br />
b ID-Spec s<strong>of</strong>tware<br />
Solutions and examples with<br />
recommended architectures in<br />
Solution guides:<br />
b airport<br />
b automative<br />
b food<br />
b retail<br />
b <strong>of</strong>fice<br />
b industrial buildings<br />
b healthcare<br />
b ...
Design support Specification help Help in <strong>installation</strong>, use &<br />
maintenance<br />
Practical data & methods<br />
through major <strong>design</strong> guides:<br />
b Electrical Installation Guide<br />
b Protection guide<br />
b Industrial <strong>electrical</strong> network<br />
<strong>design</strong> guide<br />
b ...<br />
Product presentation <strong>of</strong> technical<br />
characteristics in all Schneider<br />
Electric product Catalogues<br />
Design S<strong>of</strong>tware:<br />
b My Ecodial<br />
Ecodial s<strong>of</strong>tware provides a<br />
complete <strong>design</strong> package for<br />
LV <strong>installation</strong>s, in accordance<br />
with IEC standards and<br />
recommendations.<br />
Main features:<br />
v Create diagrams<br />
v Optimise circuit breakers curves<br />
v Determine source power<br />
v Follow step by step calculation<br />
v Print the project <strong>design</strong> file<br />
b SISPRO building<br />
b ...<br />
Drawing source files for<br />
connection, dimension, diagram,<br />
mounting & safety: CAD library<br />
Technical specification on<br />
products & solutions for tender<br />
request<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Product <strong>installation</strong> data<br />
Product how to use data<br />
Product maintenance data
<strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong><br />
<strong>installation</strong> <strong>design</strong><br />
Connection to the MV utility<br />
distribution network<br />
Connection to the LV utility<br />
distribution network<br />
MV & LV architecture selection<br />
guide<br />
LV Distribution<br />
Protection against electric<br />
shocks<br />
Sizing and protection <strong>of</strong><br />
conductors<br />
LV switchgear: functions &<br />
selection<br />
Protection against voltage<br />
surges in LV<br />
Energy Efficiency in <strong>electrical</strong><br />
distribution<br />
Power factor correction and<br />
harmonic filtering<br />
Harmonic management<br />
Characteristics <strong>of</strong> particular<br />
sources and loads<br />
Residential and other special<br />
locations<br />
EMC guidelines<br />
A<br />
B<br />
C<br />
D<br />
E<br />
F<br />
G<br />
H<br />
J<br />
K<br />
L<br />
M<br />
N<br />
P<br />
Q
A<br />
B<br />
C<br />
D<br />
E<br />
F<br />
G<br />
H<br />
<strong>General</strong> contents<br />
<strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
1 Methodology A2<br />
2 Rules and statutory regulations A4<br />
3 Installed power loads - Characteristics A10<br />
4 Power loading <strong>of</strong> an <strong>installation</strong> A15<br />
Connection to the MV utility distribution network<br />
1 Supply <strong>of</strong> power at medium voltage B2<br />
2 Procedure for the establishment <strong>of</strong> a new substation B14<br />
3 Protection aspect B16<br />
4 The consumer substation with LV metering B22<br />
5 The consumer substation with MV metering B30<br />
6 Constitution <strong>of</strong> MV/LV distribution substations B35<br />
Connection to the LV utility distribution network<br />
1 Low voltage utility distribution networks C2<br />
2 Tariffs and metering C16<br />
MV & LV architecture selection guide<br />
1 Stakes for the user D3<br />
2 Simplified architecture <strong>design</strong> process D4<br />
3 Electrical <strong>installation</strong> characteristics D7<br />
4 Technological characteristics D11<br />
5 Architecture assessment criteria D13<br />
6 Choice <strong>of</strong> architecture fundamentals D15<br />
7 Choice <strong>of</strong> architecture details D19<br />
8 Choice <strong>of</strong> equiment D24<br />
9 Recommendations for architecture optimization D26<br />
10 Glossary D29<br />
11 ID-Spec s<strong>of</strong>tware D30<br />
12 Example: <strong>electrical</strong> <strong>installation</strong> in a printworks D31<br />
LV Distribution<br />
1 Earthing schemes E2<br />
2 The <strong>installation</strong> system E15<br />
3 External influences (IEC 60364-5-51) E25<br />
Protection against electric shocks<br />
1 <strong>General</strong> F2<br />
2 Protection against direct contact F4<br />
3 Protection against indirect contact F6<br />
4 Protection <strong>of</strong> goods due to insulation fault F17<br />
5 Implementation <strong>of</strong> the TT system F19<br />
6 Implementation <strong>of</strong> the TN system F25<br />
7 Implementation <strong>of</strong> the IT system F31<br />
8 Residual current differential devices RCDs F38<br />
Sizing and protection <strong>of</strong> conductors<br />
1 <strong>General</strong> G2<br />
2 Practical method for determining the smallest allowable G7<br />
cross-sectional area <strong>of</strong> circuit conductors<br />
3 Determination <strong>of</strong> voltage drop G20<br />
4 Short-circuit current G24<br />
5 Particular cases <strong>of</strong> short-circuit current G30<br />
6 Protective earthing conductor G37<br />
7 The neutral conductor G42<br />
8 Worked example <strong>of</strong> cable calculation G46<br />
LV switchgear: functions & selection<br />
1 The basic functions <strong>of</strong> LV switchgear H2<br />
2 The switchgear H5<br />
3 Choice <strong>of</strong> switchgear H10<br />
4 Circuit breaker H11<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
J<br />
K<br />
L<br />
M<br />
N<br />
P<br />
Q<br />
<strong>General</strong> contents<br />
Protection against voltage surges in LV<br />
1 <strong>General</strong> J2<br />
2 Overvoltage protection devices J6<br />
3 Standards J11<br />
4 Choosing a protection device J14<br />
Energy Efficiency in <strong>electrical</strong> distribution<br />
1 Introduction K2<br />
2 Energy efficiency and electricity K3<br />
3 One process, several players K5<br />
4 From <strong>electrical</strong> measurement to <strong>electrical</strong> information K10<br />
5 Communication and Information System K16<br />
Power factor correction and harmonic filtering<br />
1 Reactive energy and power factor L2<br />
2 Why to improve the power factor? L5<br />
3 How to improve the power factor? L7<br />
4 Where to install power correction capacitors? L10<br />
5 How to decide the optimum level <strong>of</strong> compensation? L12<br />
6 Compensation at the terminals <strong>of</strong> a transformer L15<br />
7 Power factor correction <strong>of</strong> induction motors L18<br />
8 Example <strong>of</strong> an <strong>installation</strong> before and after power factor correction L20<br />
9 The effects <strong>of</strong> harmonics L21<br />
10 Implementation <strong>of</strong> capacitor banks L24<br />
Harmonic management<br />
1 The problem: M2<br />
Why is it necessary to detect and eliminate harmonics?<br />
2 Standards M3<br />
3 <strong>General</strong> M4<br />
4 Main effects <strong>of</strong> harmonics in <strong>installation</strong>s M6<br />
5 Essential indicators <strong>of</strong> harmonic distortion and M11<br />
measurement principles<br />
6 Measuring the indicators M14<br />
7 Detection devices M16<br />
8 Solutions to attenuate harmonics M17<br />
Characteristics <strong>of</strong> particular sources and loads<br />
1 Protection <strong>of</strong> a LV generator set and the downstream circuits N2<br />
2 Uninterruptible Power Supply Units (UPS) N11<br />
3 Protection <strong>of</strong> LV/LV transformers N24<br />
4 Lighting circuits N27<br />
5 Asynchronous motors N42<br />
Residential and other special locations<br />
1 Residential and similar premises P2<br />
2 Bathrooms and showers P8<br />
3 Recommendations applicable to special <strong>installation</strong>s and locations P12<br />
EMC guidelines<br />
1 Electrical distribution Q2<br />
2 Earthing principles and structures Q3<br />
3 Implementation Q5<br />
4 Coupling mechanism and counter-measures Q14<br />
5 Wiring recommendations Q20<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
2<br />
3<br />
4<br />
<strong>Chapter</strong> A<br />
<strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong><br />
<strong>installation</strong> <strong>design</strong><br />
Contents<br />
Methodology A2<br />
Rules and statutory regulations A4<br />
2.1 Definition <strong>of</strong> voltage ranges A4<br />
2.2 Regulations A5<br />
2.3 Standards A5<br />
2.4 Quality and safety <strong>of</strong> an <strong>electrical</strong> <strong>installation</strong> A6<br />
2.5 Initial testing <strong>of</strong> an <strong>installation</strong> A6<br />
2.6 Periodic check-testing <strong>of</strong> an <strong>installation</strong><br />
2.7 Conformity (with standards and specifications) <strong>of</strong> equipment<br />
A7<br />
used in the <strong>installation</strong> A7<br />
2.8 Environment A8<br />
Installed power loads - Characteristics A 0<br />
3.1 Induction motors<br />
3.2 Resistive-type heating appliances and incandescent lamps<br />
A10<br />
(conventional or halogen) A12<br />
Power loading <strong>of</strong> an <strong>installation</strong> A 5<br />
4.1 Installed power (kW) A15<br />
4.2 Installed apparent power (kVA) A15<br />
4.3 Estimation <strong>of</strong> actual maximum kVA demand A16<br />
4.4 Example <strong>of</strong> application <strong>of</strong> factors ku and ks A17<br />
4.5 Diversity factor A18<br />
4.6 Choice <strong>of</strong> transformer rating A19<br />
4.7 Choice <strong>of</strong> power-supply sources A20<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
A<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
A2<br />
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
B – Connection to the MV utility distribution<br />
network<br />
C - Connection to the LV utility distribution<br />
network<br />
D - MV & LV architecture selection guide<br />
E - LV Distribution<br />
F - Protection against electric shocks<br />
G - Sizing and protection <strong>of</strong> conductors<br />
H - LV switchgear: functions & selection<br />
Methodology<br />
For the best results in <strong>electrical</strong> <strong>installation</strong> <strong>design</strong> it is recommended to read all the<br />
chapters <strong>of</strong> this guide in the order in which they are presented.<br />
Listing <strong>of</strong> power demands<br />
The study <strong>of</strong> a proposed <strong>electrical</strong> <strong>installation</strong> requires an adequate understanding <strong>of</strong><br />
all governing <strong>rules</strong> and regulations.<br />
The total power demand can be calculated from the data relative to the location and<br />
power <strong>of</strong> each load, together with the knowledge <strong>of</strong> the operating modes (steady<br />
state demand, starting conditions, non simultaneous operation, etc.)<br />
From these data, the power required from the supply source and (where appropriate)<br />
the number <strong>of</strong> sources necessary for an adequate supply to the <strong>installation</strong> are<br />
readily obtained.<br />
Local information regarding tariff structures is also required to allow the best choice<br />
<strong>of</strong> connection arrangement to the power-supply network, e.g. at medium voltage or<br />
low voltage level.<br />
Service connection<br />
This connection can be made at:<br />
b Medium Voltage level<br />
A consumer-type substation will then have to be studied, built and equipped. This<br />
substation may be an outdoor or indoor <strong>installation</strong> conforming to relevant standards<br />
and regulations (the low-voltage section may be studied separately if necessary).<br />
Metering at medium-voltage or low-voltage is possible in this case.<br />
b Low Voltage level<br />
The <strong>installation</strong> will be connected to the local power network and will (necessarily) be<br />
metered according to LV tariffs.<br />
Electrical Distribution architecture<br />
The whole <strong>installation</strong> distribution network is studied as a complete system.<br />
A selection guide is proposed for determination <strong>of</strong> the most suitable architecture.<br />
MV/LV main distribution and LV power distribution levels are covered.<br />
Neutral earthing arrangements are chosen according to local regulations, constraints<br />
related to the power-supply, and to the type <strong>of</strong> loads.<br />
The distribution equipment (panelboards, switchgears, circuit connections, ...) are<br />
determined from building plans and from the location and grouping <strong>of</strong> loads.<br />
The type <strong>of</strong> premises and allocation can influence their immunity to external<br />
disturbances.<br />
Protection against electric shocks<br />
The earthing system (TT, IT or TN) having been previously determined, then the<br />
appropriate protective devices must be implemented in order to achieve protection<br />
against hazards <strong>of</strong> direct or indirect contact.<br />
Circuits and switchgear<br />
Each circuit is then studied in detail. From the rated currents <strong>of</strong> the loads, the level<br />
<strong>of</strong> short-circuit current, and the type <strong>of</strong> protective device, the cross-sectional area<br />
<strong>of</strong> circuit conductors can be determined, taking into account the nature <strong>of</strong> the<br />
cableways and their influence on the current rating <strong>of</strong> conductors.<br />
Before adopting the conductor size indicated above, the following requirements must<br />
be satisfied:<br />
b The voltage drop complies with the relevant standard<br />
b Motor starting is satisfactory<br />
b Protection against electric shock is assured<br />
The short-circuit current Isc is then determined, and the thermal and electrodynamic<br />
withstand capability <strong>of</strong> the circuit is checked.<br />
These calculations may indicate that it is necessary to use a conductor size larger<br />
than the size originally chosen.<br />
The performance required by the switchgear will determine its type and<br />
characteristics.<br />
The use <strong>of</strong> cascading techniques and the discriminative operation <strong>of</strong> fuses and<br />
tripping <strong>of</strong> circuit breakers are examined.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
J – Protection against voltage surges in LV<br />
K – Energy efficiency in <strong>electrical</strong> distribution<br />
L - Power factor correction and harmonic filtering<br />
M - Harmonic management<br />
N - Characteristics <strong>of</strong> particular sources and<br />
loads<br />
P - Residential and other special locations<br />
Q - EMC guideline<br />
(1) Ecodial is a Merlin Gerin product and is available in French<br />
and English versions.<br />
Methodology<br />
Protection against overvoltages<br />
Direct or indirect lightning strokes can damage <strong>electrical</strong> equipment at a distance<br />
<strong>of</strong> several kilometers. Operating voltage surges, transient and industrial frequency<br />
over-voltage can also produce the same consequences.The effects are examined<br />
and solutions are proposed.<br />
Energy efficiency in electrial distribution<br />
Implementation <strong>of</strong> measuring devices with an adequate communication system<br />
within the <strong>electrical</strong> <strong>installation</strong> can produce high benefits for the user or owner:<br />
reduced power consumption, reduced cost <strong>of</strong> energy, better use <strong>of</strong> <strong>electrical</strong><br />
equipment.<br />
Reactive energy<br />
The power factor correction within <strong>electrical</strong> <strong>installation</strong>s is carried out locally,<br />
globally or as a combination <strong>of</strong> both methods.<br />
Harmonics<br />
Harmonics in the network affect the quality <strong>of</strong> energy and are at the origin <strong>of</strong> many<br />
disturbances as overloads, vibrations, ageing <strong>of</strong> equipment, trouble <strong>of</strong> sensitive<br />
equipment, <strong>of</strong> local area networks, telephone networks. This chapter deals with the<br />
origins and the effects <strong>of</strong> harmonics and explain how to measure them and present<br />
the solutions.<br />
Particular supply sources and loads<br />
Particular items or equipment are studied:<br />
b Specific sources such as alternators or inverters<br />
b Specific loads with special characteristics, such as induction motors, lighting<br />
circuits or LV/LV transformers<br />
b Specific systems, such as direct-current networks<br />
Generic applications<br />
Certain premises and locations are subject to particularly strict regulations: the most<br />
common example being residential dwellings.<br />
EMC Guidelines<br />
Some basic <strong>rules</strong> must be followed in order to ensure Electromagnetic Compatibility.<br />
Non observance <strong>of</strong> these <strong>rules</strong> may have serious consequences in the operation <strong>of</strong><br />
the <strong>electrical</strong> <strong>installation</strong>: disturbance <strong>of</strong> communication systems, nuisance tripping<br />
<strong>of</strong> protection devices, and even destruction <strong>of</strong> sensitive devices.<br />
Ecodial s<strong>of</strong>tware<br />
Ecodial s<strong>of</strong>tware (1) provides a complete <strong>design</strong> package for LV <strong>installation</strong>s, in<br />
accordance with IEC standards and recommendations.<br />
The following features are included:<br />
b Construction <strong>of</strong> one-line diagrams<br />
b Calculation <strong>of</strong> short-circuit currents<br />
b Calculation <strong>of</strong> voltage drops<br />
b Optimization <strong>of</strong> cable sizes<br />
b Required ratings <strong>of</strong> switchgear and fusegear<br />
b Discrimination <strong>of</strong> protective devices<br />
b Recommendations for cascading schemes<br />
b Verification <strong>of</strong> the protection <strong>of</strong> people<br />
b Comprehensive print-out <strong>of</strong> the foregoing calculated <strong>design</strong> data<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
A3<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
A<br />
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
2 Rules and statutory regulations<br />
Low-voltage <strong>installation</strong>s are governed by a number <strong>of</strong> regulatory and advisory texts,<br />
which may be classified as follows:<br />
b Statutory regulations (decrees, factory acts,etc.)<br />
b Codes <strong>of</strong> practice, regulations issued by pr<strong>of</strong>essional institutions, job specifications<br />
b National and international standards for <strong>installation</strong>s<br />
b National and international standards for products<br />
2.1 Definition <strong>of</strong> voltage ranges<br />
IEC voltage standards and recommendations<br />
Three-phase four-wire or three-wire systems Single-phase three-wire systems<br />
Nominal voltage (V) Nominal voltage (V)<br />
50 Hz 60 Hz 60 Hz<br />
– 120/208 120/240<br />
– 240 –<br />
230/400 (1) 277/480 –<br />
400/690 (1) 480 –<br />
– 347/600 –<br />
1000 600 –<br />
(1) The nominal voltage <strong>of</strong> existing 220/380 V and 240/415 V systems shall evolve<br />
toward the recommended value <strong>of</strong> 230/400 V. The transition period should be as short<br />
as possible and should not exceed the year 2003. During this period, as a first step, the<br />
electricity supply authorities <strong>of</strong> countries having 220/380 V systems should bring the<br />
voltage within the range 230/400 V +6 %, -10 % and those <strong>of</strong> countries having<br />
240/415 V systems should bring the voltage within the range 230/400 V +10 %,<br />
-6 %. At the end <strong>of</strong> this transition period, the tolerance <strong>of</strong> 230/400 V ± 10 % should<br />
have been achieved; after this the reduction <strong>of</strong> this range will be considered. All the<br />
above considerations apply also to the present 380/660 V value with respect to the<br />
recommended value 400/690 V.<br />
Fig. A1 : Standard voltages between 100 V and 1000 V (IEC 60038 Edition 6.2 2002-07)<br />
Series I Series II<br />
Highest voltage Nominal system Highest voltage Nominal system<br />
for equipment (kV) voltage (kV) for equipment (kV) voltage (kV)<br />
3.6 (1) 3.3 (1) 3 (1) 4.40 (1) 4.16 (1)<br />
7.2 (1) 6.6 (1) 6 (1) – –<br />
12 11 10 – –<br />
– – – 13.2 (2) 12.47 (2)<br />
– – – 13.97 (2) 13.2 (2)<br />
– – – 14.52 (1) 13.8 (1)<br />
(17.5) – (15) – –<br />
24 22 20 – –<br />
– – – 26.4 (2) 24.94 (2)<br />
36 (3) 33 (3) – – –<br />
– – – 36.5 34.5<br />
40.5 (3) – 35 (3) – –<br />
These systems are generally three-wire systems unless otherwise indicated.<br />
The values indicated are voltages between phases.<br />
The values indicated in parentheses should be considered as non-preferred values. It is<br />
recommended that these values should not be used for new systems to be constructed<br />
in future.<br />
Note 1: It is recommended that in any one country the ratio between two adjacent<br />
nominal voltages should be not less than two.<br />
Note 2: In a normal system <strong>of</strong> Series I, the highest voltage and the lowest voltage do<br />
not differ by more than approximately ±10 % from the nominal voltage <strong>of</strong> the system.<br />
In a normal system <strong>of</strong> Series II, the highest voltage does not differ by more then +5 %<br />
and the lowest voltage by more than -10 % from the nominal voltage <strong>of</strong> the system.<br />
(1) These values should not be used for public distribution systems.<br />
(2) These systems are generally four-wire systems.<br />
(3) The unification <strong>of</strong> these values is under consideration.<br />
Fig. A2 : Standard voltages above 1 kV and not exceeding 35 kV<br />
(IEC 60038 Edition 6.2 2002-07)<br />
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2 Rules and statutory regulations<br />
2.2 Regulations<br />
In most countries, <strong>electrical</strong> <strong>installation</strong>s shall comply with more than one set <strong>of</strong><br />
regulations, issued by National Authorities or by recognized private bodies. It is<br />
essential to take into account these local constraints before starting the <strong>design</strong>.<br />
2.3 Standards<br />
This Guide is based on relevant IEC standards, in particular IEC 60364. IEC 60364<br />
has been established by medical and engineering experts <strong>of</strong> all countries in the<br />
world comparing their experience at an international level. Currently, the safety<br />
principles <strong>of</strong> IEC 60364 and 60479-1 are the fundamentals <strong>of</strong> most <strong>electrical</strong><br />
standards in the world (see table below and next page).<br />
IEC 60038 Standard voltages<br />
IEC 60076-2 Power transformers - Temperature rise<br />
IEC 60076-3 Power transformers - Insulation levels, dielectric tests and external clearances in air<br />
IEC 60076- Power transformers - Ability to withstand short-circuit<br />
IEC 60076-10 Power transformers - Determination <strong>of</strong> sound levels<br />
IEC 601 6 Semiconductor convertors - <strong>General</strong> requirements and line commutated convertors<br />
IEC 602 Electrical relays<br />
IEC 6026 -1 High-voltage switches - High-voltage switches for rated voltages above 1 kV and less than 52 kV<br />
IEC 60269-1 Low-voltage fuses - <strong>General</strong> requirements<br />
IEC 60269-2 Low-voltage fuses - Supplementary requirements for fuses for use by unskilled persons (fuses mainly for household and similar applications)<br />
IEC 60282-1 High-voltage fuses - Current-limiting fuses<br />
IEC 60287-1-1 Electric cables - Calculation <strong>of</strong> the current rating - Current rating equations (100% load factor) and calculation <strong>of</strong> losses - <strong>General</strong><br />
IEC 6036 Electrical <strong>installation</strong>s <strong>of</strong> buildings<br />
IEC 6036 -1 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Fundamental principles<br />
IEC 6036 - - 1 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Protection for safety - Protection against electric shock<br />
IEC 6036 - - 2 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Protection for safety - Protection against thermal effects<br />
IEC 6036 - - 3 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Protection for safety - Protection against overcurrent<br />
IEC 6036 - - Electrical <strong>installation</strong>s <strong>of</strong> buildings - Protection for safety - Protection against electromagnetic and voltage disrurbance<br />
IEC 6036 - - 1 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Selection and erection <strong>of</strong> <strong>electrical</strong> equipment - Common <strong>rules</strong><br />
IEC 6036 - - 2 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Selection and erection <strong>of</strong> <strong>electrical</strong> equipment - Wiring systems<br />
IEC 6036 - - 3 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Selection and erection <strong>of</strong> <strong>electrical</strong> equipment - Isolation, switching and control<br />
IEC 6036 - - Electrical <strong>installation</strong>s <strong>of</strong> buildings - Selection and erection <strong>of</strong> <strong>electrical</strong> equipment - Earthing arrangements<br />
IEC 6036 - - Electrical <strong>installation</strong>s <strong>of</strong> buildings - Selection and erection <strong>of</strong> <strong>electrical</strong> equipment - Other equipments<br />
IEC 6036 -6-61 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Verification and testing - Initial verification<br />
IEC 6036 -7-701 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Locations containing a bath tub or shower basin<br />
IEC 6036 -7-702 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Swimming pools and other basins<br />
IEC 6036 -7-703 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Locations containing sauna heaters<br />
IEC 6036 -7-70 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Construction and demolition site <strong>installation</strong>s<br />
IEC 6036 -7-70 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Electrical <strong>installation</strong>s <strong>of</strong> agricultural and horticultural<br />
premises<br />
IEC 6036 -7-706 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Restrictive conducting locations<br />
IEC 6036 -7-707 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Earthing requirements for the <strong>installation</strong> <strong>of</strong> data<br />
processing equipment<br />
IEC 6036 -7-708 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Electrical <strong>installation</strong>s in caravan parks and caravans<br />
IEC 6036 -7-709 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Marinas and pleasure craft<br />
IEC 6036 -7-710 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Medical locations<br />
IEC 6036 -7-711 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Exhibitions, shows and stands<br />
IEC 6036 -7-712 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Solar photovoltaic (PV) power supply systems<br />
IEC 6036 -7-713 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Furniture<br />
IEC 6036 -7-71 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - External lighting <strong>installation</strong>s<br />
IEC 6036 -7-71 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Extra-low-voltage lighting <strong>installation</strong>s<br />
IEC 6036 -7-717 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Mobile or transportable units<br />
IEC 6036 -7-7 0 Electrical <strong>installation</strong>s <strong>of</strong> buildings - Requirements for special <strong>installation</strong>s or locations - Temporary <strong>electrical</strong> <strong>installation</strong>s for structures,<br />
amusement devices and booths at fairgrounds, amusement parks and circuses<br />
IEC 60 27 High-voltage alternating current circuit-breakers<br />
IEC 60 39-1 Low-voltage switchgear and controlgear assemblies - Type-tested and partially type-tested assemblies<br />
IEC 60 39-2 Low-voltage switchgear and controlgear assemblies - Particular requirements for busbar trunking systems (busways)<br />
IEC 60 39-3 Low-voltage switchgear and controlgear assemblies - Particular requirements for low-voltage switchgear and controlgear assemblies intended to<br />
be installed in places where unskilled persons have access for their use - Distribution boards<br />
IEC 60 39- Low-voltage switchgear and controlgear assemblies - Particular requirements for assemblies for construction sites (ACS)<br />
IEC 60 6 Basic and safety principles for man-machine interface, marking and identification - Identification <strong>of</strong> conductors by colours or numerals<br />
IEC 60 39- Low-voltage switchgear and controlgear assemblies - Particular requirements for assemblies intended to be installed outdoors in public places<br />
- Cable distribution cabinets (CDCs)<br />
IEC 60 79-1 Effects <strong>of</strong> current on human beings and livestock - <strong>General</strong> aspects<br />
IEC 60 79-2 Effects <strong>of</strong> current on human beings and livestock - Special aspects<br />
IEC 60 79-3 Effects <strong>of</strong> current on human beings and livestock - Effects <strong>of</strong> currents passing through the body <strong>of</strong> livestock<br />
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2 Rules and statutory regulations<br />
IEC 60 29 Degrees <strong>of</strong> protection provided by enclosures (IP code)<br />
IEC 606 Spécification for high-voltage fuse-links for motor circuit applications<br />
IEC 6066 Insulation coordination for equipment within low-voltage systems<br />
IEC 6071 Dimensions <strong>of</strong> low-voltage switchgear and controlgear. Standardized mounting on rails for mechanical support <strong>of</strong> <strong>electrical</strong> devices in switchgear<br />
and controlgear <strong>installation</strong>s.<br />
IEC 6072 Short-circuit temperature limits <strong>of</strong> electric cables with rated voltages <strong>of</strong> 1 kV (Um = 1.2 kV) and 3 kV (Um = 3.6 kV)<br />
IEC 607 <strong>General</strong> requirements for residual current operated protective devices<br />
IEC 60787 Application guide for the selection <strong>of</strong> fuse-links <strong>of</strong> high-voltage fuses for transformer circuit application<br />
IEC 60831 Shunt power capacitors <strong>of</strong> the self-healing type for AC systems having a rated voltage up to and including 1000 V - <strong>General</strong> - Performance, testing<br />
and rating - Safety requirements - Guide for <strong>installation</strong> and operation<br />
IEC 609 7-1 Low-voltage switchgear and controlgear - <strong>General</strong> <strong>rules</strong><br />
IEC 609 7-2 Low-voltage switchgear and controlgear - Circuit-breakers<br />
IEC 609 7-3 Low-voltage switchgear and controlgear - Switches, disconnectors, switch-disconnectors and fuse-combination units<br />
IEC 609 7- -1 Low-voltage switchgear and controlgear - Contactors and motor-starters - Electromechanical contactors and motor-starters<br />
IEC 609 7-6-1 Low-voltage switchgear and controlgear - Multiple function equipment - Automatic transfer switching equipment<br />
IEC 61000 Electromagnetic compatibility (EMC)<br />
IEC 611 0 Protection against electric shocks - common aspects for <strong>installation</strong> and equipment<br />
IEC 61 7-1 Electrical safety in low-voltage distribution systems up to 1000 V AC and 1500 V DC - Equipment for testing, measuring or monitoring <strong>of</strong> protective<br />
measures - <strong>General</strong> requirements<br />
IEC 61 7-8 Electrical safety in low-voltage distribution systems up to 1000 V AC and 1500 V DC - Equipment for testing, measuring or monitoring <strong>of</strong> protective<br />
measures<br />
IEC 61 7-9 Electrical safety in low-voltage distribution systems up to 1000 V AC and 1500 V DC - Equipment for insulation fault location in IT systems<br />
IEC 61 8-2-6 Safety <strong>of</strong> power transformers, power supply units and similar - Particular requirements for safety isolating transformers for general use<br />
IEC 62271-1 Common specifications for high-voltage switchgear and controlgear standards<br />
IEC 62271-100 High-voltage switchgear and controlgear - High-voltage alternating-current circuit-breakers<br />
IEC 62271-102 High-voltage switchgear and controlgear - Alternating current disconnectors and earthing switches<br />
IEC 62271-10 High-voltage switchgear and controlgear - Alternating current switch-fuse combinations<br />
IEC 62271-200 High-voltage switchgear and controlgear - Alternating current metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to<br />
and including 52 kV<br />
IEC 62271-202 High-voltage/low voltage prefabricated substations<br />
2. Quality and safety <strong>of</strong> an <strong>electrical</strong> <strong>installation</strong><br />
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(Concluded)<br />
In so far as control procedures are respected, quality and safety will be assured<br />
only if:<br />
b The initial checking <strong>of</strong> conformity <strong>of</strong> the <strong>electrical</strong> <strong>installation</strong> with the standard and<br />
regulation has been achieved<br />
b The <strong>electrical</strong> equipment comply with standards<br />
b The periodic checking <strong>of</strong> the <strong>installation</strong> recommended by the equipment<br />
manufacturer is respected.<br />
2. Initial testing <strong>of</strong> an <strong>installation</strong><br />
Before a utility will connect an <strong>installation</strong> to its supply network, strict precommissioning<br />
<strong>electrical</strong> tests and visual inspections by the authority, or by its<br />
appointed agent, must be satisfied.<br />
These tests are made according to local (governmental and/or institutional)<br />
regulations, which may differ slightly from one country to another. The principles <strong>of</strong><br />
all such regulations however, are common, and are based on the observance <strong>of</strong><br />
rigorous safety <strong>rules</strong> in the <strong>design</strong> and realization <strong>of</strong> the <strong>installation</strong>.<br />
IEC 60364-6-61 and related standards included in this guide are based on an<br />
international consensus for such tests, intended to cover all the safety measures and<br />
approved <strong>installation</strong> practices normally required for residential, commercial and (the<br />
majority <strong>of</strong>) industrial buildings. Many industries however have additional regulations<br />
related to a particular product (petroleum, coal, natural gas, etc.). Such additional<br />
requirements are beyond the scope <strong>of</strong> this guide.<br />
The pre-commissioning <strong>electrical</strong> tests and visual-inspection checks for <strong>installation</strong>s<br />
in buildings include, typically, all <strong>of</strong> the following:<br />
b Insulation tests <strong>of</strong> all cable and wiring conductors <strong>of</strong> the fixed <strong>installation</strong>, between<br />
phases and between phases and earth<br />
b Continuity and conductivity tests <strong>of</strong> protective, equipotential and earth-bonding<br />
conductors<br />
b Resistance tests <strong>of</strong> earthing electrodes with respect to remote earth<br />
b Verification <strong>of</strong> the proper operation <strong>of</strong> the interlocks, if any<br />
b Check <strong>of</strong> allowable number <strong>of</strong> socket-outlets per circuit
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
Conformity <strong>of</strong> equipment with the relevant<br />
standards can be attested in several ways<br />
2 Rules and statutory regulations<br />
b Cross-sectional-area check <strong>of</strong> all conductors for adequacy at the short-circuit<br />
levels prevailing, taking account <strong>of</strong> the associated protective devices, materials and<br />
<strong>installation</strong> conditions (in air, conduit, etc.)<br />
b Verification that all exposed- and extraneous metallic parts are properly earthed<br />
(where appropriate)<br />
b Check <strong>of</strong> clearance distances in bathrooms, etc.<br />
These tests and checks are basic (but not exhaustive) to the majority <strong>of</strong> <strong>installation</strong>s,<br />
while numerous other tests and <strong>rules</strong> are included in the regulations to cover<br />
particular cases, for example: TN-, TT- or IT-earthed <strong>installation</strong>s, <strong>installation</strong>s based<br />
on class 2 insulation, SELV circuits, and special locations, etc.<br />
The aim <strong>of</strong> this guide is to draw attention to the particular features <strong>of</strong> different types<br />
<strong>of</strong> <strong>installation</strong>, and to indicate the essential <strong>rules</strong> to be observed in order to achieve<br />
a satisfactory level <strong>of</strong> quality, which will ensure safe and trouble-free performance.<br />
The methods recommended in this guide, modified if necessary to comply with any<br />
possible variation imposed by a utility, are intended to satisfy all precommissioning<br />
test and inspection requirements.<br />
2.6 Periodic check-testing <strong>of</strong> an <strong>installation</strong><br />
In many countries, all industrial and commercial-building <strong>installation</strong>s, together with<br />
<strong>installation</strong>s in buildings used for public gatherings, must be re-tested periodically by<br />
authorized agents.<br />
Figure A3 shows the frequency <strong>of</strong> testing commonly prescribed according to the<br />
kind <strong>of</strong> <strong>installation</strong> concerned.<br />
Type <strong>of</strong> <strong>installation</strong> Testing<br />
frequency<br />
Installations which b Locations at which a risk <strong>of</strong> degradation, Annually<br />
require the protection fire or explosion exists<br />
<strong>of</strong> employees b Temporary <strong>installation</strong>s at worksites<br />
b Locations at which MV <strong>installation</strong>s exist<br />
b Restrictive conducting locations<br />
where mobile equipment is used<br />
Other cases Every 3 years<br />
Installations in buildings According to the type <strong>of</strong> establishment From one to<br />
used for public gatherings, and its capacity for receiving the public three years<br />
where protection against<br />
the risks <strong>of</strong> fire and panic<br />
are required<br />
Residential According to local regulations<br />
Fig A3 : Frequency <strong>of</strong> check-tests commonly recommended for an <strong>electrical</strong> <strong>installation</strong><br />
2.7 Conformity (with standards and specifications)<br />
<strong>of</strong> equipment used in the <strong>installation</strong><br />
Attestation <strong>of</strong> conformity<br />
The conformity <strong>of</strong> equipment with the relevant standards can be attested:<br />
b By an <strong>of</strong>ficial mark <strong>of</strong> conformity granted by the certification body concerned, or<br />
b By a certificate <strong>of</strong> conformity issued by a certification body, or<br />
b By a declaration <strong>of</strong> conformity from the manufacturer<br />
The first two solutions are generally not available for high voltage equipment.<br />
Declaration <strong>of</strong> conformity<br />
Where the equipment is to be used by skilled or instructed persons, the<br />
manufacturer’s declaration <strong>of</strong> conformity (included in the technical documentation),<br />
is generally recognized as a valid attestation. Where the competence <strong>of</strong> the<br />
manufacturer is in doubt, a certificate <strong>of</strong> conformity can reinforce the manufacturer’s<br />
declaration.<br />
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2 Rules and statutory regulations<br />
Note: CE marking<br />
In Europe, the European directives require the manufacturer or his authorized<br />
representative to affix the CE marking on his own responsibility. It means that:<br />
b The product meets the legal requirements<br />
b It is presumed to be marketable in Europe<br />
The CE marking is neither a mark <strong>of</strong> origin nor a mark <strong>of</strong> conformity.<br />
Mark <strong>of</strong> conformity<br />
Marks <strong>of</strong> conformity are affixed on appliances and equipment generally used by<br />
ordinary non instructed people (e.g in the field <strong>of</strong> domestic appliances). A mark <strong>of</strong><br />
conformity is delivered by certification body if the equipment meet the requirements<br />
from an applicable standard and after verification <strong>of</strong> the manufacturer’s quality<br />
management system.<br />
Certification <strong>of</strong> Quality<br />
The standards define several methods <strong>of</strong> quality assurance which correspond to<br />
different situations rather than to different levels <strong>of</strong> quality.<br />
Assurance<br />
A laboratory for testing samples cannot certify the conformity <strong>of</strong> an entire production<br />
run: these tests are called type tests. In some tests for conformity to standards,<br />
the samples are destroyed (tests on fuses, for example).<br />
Only the manufacturer can certify that the fabricated products have, in fact,<br />
the characteristics stated.<br />
Quality assurance certification is intended to complete the initial declaration or<br />
certification <strong>of</strong> conformity.<br />
As pro<strong>of</strong> that all the necessary measures have been taken for assuring the quality <strong>of</strong><br />
production, the manufacturer obtains certification <strong>of</strong> the quality control system which<br />
monitors the fabrication <strong>of</strong> the product concerned. These certificates are issued<br />
by organizations specializing in quality control, and are based on the international<br />
standard ISO 9001: 2000.<br />
These standards define three model systems <strong>of</strong> quality assurance control<br />
corresponding to different situations rather than to different levels <strong>of</strong> quality:<br />
b Model 3 defines assurance <strong>of</strong> quality by inspection and checking <strong>of</strong> final products.<br />
b Model 2 includes, in addition to checking <strong>of</strong> the final product, verification <strong>of</strong> the<br />
manufacturing process. For example, this method is applied, to the manufacturer <strong>of</strong><br />
fuses where performance characteristics cannot be checked without destroying the<br />
fuse.<br />
b Model 1 corresponds to model 2, but with the additional requirement that the<br />
quality <strong>of</strong> the <strong>design</strong> process must be rigorously scrutinized; for example, where it is<br />
not intended to fabricate and test a prototype (case <strong>of</strong> a custom-built product made to<br />
specification).<br />
2.8 Environment<br />
Environmental management systems can be certified by an independent body if they<br />
meet requirements given in ISO 14001. This type <strong>of</strong> certification mainly concerns<br />
industrial settings but can also be granted to places where products are <strong>design</strong>ed.<br />
A product environmental <strong>design</strong> sometimes called “eco-<strong>design</strong>” is an approach <strong>of</strong><br />
sustainable development with the objective <strong>of</strong> <strong>design</strong>ing products/services best<br />
meeting the customers’ requirements while reducing their environmental impact<br />
over their whole life cycle. The methodologies used for this purpose lead to choose<br />
equipment’s architecture together with components and materials taking into account<br />
the influence <strong>of</strong> a product on the environment along its life cycle (from extraction <strong>of</strong><br />
raw materials to scrap) i.e. production, transport, distribution, end <strong>of</strong> life etc.<br />
In Europe two Directives have been published, they are called:<br />
b RoHS Directive (Restriction <strong>of</strong> Hazardous Substances) coming into force on<br />
July 2006 (the coming into force was on February 13th , 2003, and the application<br />
date is July 1st , 2006) aims to eliminate from products six hazardous substances:<br />
lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB) or<br />
polybrominated diphenyl ethers (PBDE).<br />
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2 Rules and statutory regulations<br />
b WEEE Directive (Waste <strong>of</strong> Electrical and Electronic Equipment) coming into<br />
force in August 2005 (the coming into force was on February 13 th , 2003, and<br />
the application date is August 13 th , 2005) in order to master the end <strong>of</strong> life and<br />
treatments for household and non household equipment.<br />
In other parts <strong>of</strong> the world some new legislation will follow the same objectives.<br />
In addition to manufacturers action in favour <strong>of</strong> products eco-<strong>design</strong>, the contribution<br />
<strong>of</strong> the whole <strong>electrical</strong> <strong>installation</strong> to sustainable development can be significantly<br />
improved through the <strong>design</strong> <strong>of</strong> the <strong>installation</strong>. Actually, it has been shown that an<br />
optimised <strong>design</strong> <strong>of</strong> the <strong>installation</strong>, taking into account operation conditions, MV/LV<br />
substations location and distribution structure (switchboards, busways, cables),<br />
can reduce substantially environmental impacts (raw material depletion, energy<br />
depletion, end <strong>of</strong> life)<br />
See chapter D about location <strong>of</strong> the substation and the main LV switchboard.<br />
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An examination <strong>of</strong> the actual apparentpower<br />
demands <strong>of</strong> different loads: a<br />
necessary preliminary step in the <strong>design</strong> <strong>of</strong> a<br />
LV <strong>installation</strong><br />
The nominal power in kW (Pn) <strong>of</strong> a motor<br />
indicates its rated equivalent mechanical power<br />
output.<br />
The apparent power in kVA (Pa) supplied to<br />
the motor is a function <strong>of</strong> the output, the motor<br />
efficiency and the power factor.<br />
Pa = Pn<br />
ηcosϕ 3 Installed power loads -<br />
Characteristics<br />
The examination <strong>of</strong> actual values <strong>of</strong> apparent-power required by each load enables<br />
the establishment <strong>of</strong>:<br />
b A declared power demand which determines the contract for the supply <strong>of</strong> energy<br />
b The rating <strong>of</strong> the MV/LV transformer, where applicable (allowing for expected<br />
increased load)<br />
b Levels <strong>of</strong> load current at each distribution board<br />
3.1 Induction motors<br />
Current demand<br />
The full-load current Ia supplied to the motor is given by the following formulae:<br />
b 3-phase motor: Ia = Pn x 1,000 / (√3 x U x η x cos ϕ)<br />
b 1-phase motor: Ia = Pn x 1,000 / (U x η x cos ϕ)<br />
where<br />
Ia: current demand (in amps)<br />
Pn: nominal power (in kW)<br />
U: voltage between phases for 3-phase motors and voltage between the terminals<br />
for single-phase motors (in volts). A single-phase motor may be connected phase-toneutral<br />
or phase-to-phase.<br />
η: per-unit efficiency, i.e. output kW / input kW<br />
cos ϕ: power factor, i.e. kW input / kVA input<br />
Subtransient current and protection setting<br />
b Subtransient current peak value can be very high ; typical value is about 12<br />
to 15 times the rms rated value Inm. Sometimes this value can reach 25 times Inm.<br />
b Merlin Gerin circuit-breakers, Telemecanique contactors and thermal relays are<br />
<strong>design</strong>ed to withstand motor starts with very high subtransient current (subtransient<br />
peak value can be up to 19 times the rms rated value Inm).<br />
b If unexpected tripping <strong>of</strong> the overcurrent protection occurs during starting, this<br />
means the starting current exceeds the normal limits. As a result, some maximum<br />
switchgear withstands can be reached, life time can be reduced and even some<br />
devices can be destroyed. In order to avoid such a situation, oversizing <strong>of</strong> the<br />
switchgear must be considered.<br />
b Merlin Gerin and Telemecanique switchgears are <strong>design</strong>ed to ensure the<br />
protection <strong>of</strong> motor starters against short-circuits. According to the risk, tables show<br />
the combination <strong>of</strong> circuit-breaker, contactor and thermal relay to obtain type 1 or<br />
type 2 coordination (see chapter N).<br />
Motor starting current<br />
Although high efficiency motors can be found on the market, in practice their starting<br />
currents are roughly the same as some <strong>of</strong> standard motors.<br />
The use <strong>of</strong> start-delta starter, static s<strong>of</strong>t start unit or variable speed drive allows to<br />
reduce the value <strong>of</strong> the starting current (Example : 4 Ia instead <strong>of</strong> 7.5 Ia).<br />
Compensation <strong>of</strong> reactive-power (kvar) supplied to induction motors<br />
It is generally advantageous for technical and financial reasons to reduce the current<br />
supplied to induction motors. This can be achieved by using capacitors without<br />
affecting the power output <strong>of</strong> the motors.<br />
The application <strong>of</strong> this principle to the operation <strong>of</strong> induction motors is generally<br />
referred to as “power-factor improvement” or “power-factor correction”.<br />
As discussed in chapter L, the apparent power (kVA) supplied to an induction motor<br />
can be significantly reduced by the use <strong>of</strong> shunt-connected capacitors. Reduction<br />
<strong>of</strong> input kVA means a corresponding reduction <strong>of</strong> input current (since the voltage<br />
remains constant).<br />
Compensation <strong>of</strong> reactive-power is particularly advised for motors that operate for<br />
long periods at reduced power.<br />
kW input<br />
As noted above cos � = so so that a kVA input reduction in will kVA increase input will<br />
kVA input<br />
increase (i.e. improve) (i.e. improve) the value the <strong>of</strong> cos value ϕ. <strong>of</strong> cos<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
3 Installed power loads -<br />
Characteristics<br />
The current supplied to the motor, after power-factor correction, is given by:<br />
cos �<br />
I=<br />
Ia<br />
cos �'<br />
where cos �ϕ<br />
is the power factor before compensation and cos ϕ’ is the power factor<br />
after compensation, Ia being the original current.<br />
Figure A4 below shows, in function <strong>of</strong> motor rated power, standard motor current<br />
values for several voltage supplies.<br />
kW hp 230 V 380 - 400 V 440 - 500 V 690 V<br />
415 V 480 V<br />
A A A A A A<br />
0.18 - 1.0 - 0.6 - 0.48 0.35<br />
0.25 - 1.5 - 0.85 - 0.68 0.49<br />
0.37 - 1.9 - 1.1 - 0.88 0.64<br />
- 1/2 - 1.3 - 1.1 - -<br />
0.55 - 2.6 - 1.5 - 1.2 0.87<br />
- 3/4 - 1.8 - 1.6 - -<br />
- 1 - 2.3 - 2.1 - -<br />
0.75 - 3.3 - 1.9 - 1.5 1.1<br />
1.1 - 4.7 - 2.7 - 2.2 1.6<br />
- 1-1/2 - 3.3 - 3.0 - -<br />
- 2 - 4.3 - 3.4 - -<br />
1.5 - 6.3 - 3.6 - 2.9 2.1<br />
2.2 - 8.5 - 4.9 - 3.9 2.8<br />
- 3 - 6.1 - 4.8 - -<br />
3.0 - 11.3 - 6.5 - 5.2 3.8<br />
3.7 - - - - - - -<br />
4 - 15 9.7 8.5 7.6 6.8 4.9<br />
5.5 - 20 - 11.5 - 9.2 6.7<br />
- 7-1/2 - 14.0 - 11.0 - -<br />
- 10 - 18.0 - 14.0 - -<br />
7.5 - 27 - 15.5 - 12.4 8.9<br />
11 - 38.0 - 22.0 - 17.6 12.8<br />
- 15 - 27.0 - 21.0 - -<br />
- 20 - 34.0 - 27.0 - -<br />
15 - 51 - 29 - 23 17<br />
18.5 - 61 - 35 - 28 21<br />
- 25 - 44 - 34 -<br />
22 - 72 - 41 - 33 24<br />
- 30 - 51 - 40 - -<br />
- 40 - 66 - 52 - -<br />
30 - 96 - 55 - 44 32<br />
37 - 115 - 66 - 53 39<br />
- 50 - 83 - 65 - -<br />
- 60 - 103 - 77 - -<br />
45 - 140 - 80 - 64 47<br />
55 - 169 - 97 - 78 57<br />
- 75 - 128 - 96 - -<br />
- 100 - 165 - 124 - -<br />
75 - 230 - 132 - 106 77<br />
90 - 278 - 160 - 128 93<br />
- 125 - 208 - 156 - -<br />
110 - 340 - 195 156 113<br />
- 150 - 240 - 180 - -<br />
132 - 400 - 230 - 184 134<br />
- 200 - 320 - 240 - -<br />
150 - - - - - - -<br />
160 - 487 - 280 - 224 162<br />
185 - - - - - - -<br />
- 250 - 403 - 302 - -<br />
200 - 609 - 350 - 280 203<br />
220 - - - - - - -<br />
- 300 - 482 - 361 - -<br />
250 - 748 - 430 - 344 250<br />
280 - - - - - - -<br />
- 350 - 560 - 414 - -<br />
- 400 - 636 - 474 - -<br />
300 - - - - - - -<br />
Fig. A4 : Rated operational power and currents (continued on next page)<br />
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A12<br />
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A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
3 Installed power loads -<br />
Characteristics<br />
kW hp 230 V 380 - 400 V 440 - 500 V 690 V<br />
415 V 480 V<br />
A A A A A A<br />
315 - 940 - 540 - 432 313<br />
- 540 - - - 515 - -<br />
335 - - - - - - -<br />
355 - 1061 - 610 - 488 354<br />
- 500 - 786 - 590 - -<br />
375 - - - - - - -<br />
400 - 1200 - 690 - 552 400<br />
425 - - - - - - -<br />
450 - - - - - - -<br />
475 - - - - - - -<br />
500 - 1478 - 850 - 680 493<br />
530 - - - - - - -<br />
560 - 1652 - 950 - 760 551<br />
600 - - - - - - -<br />
630 - 1844 - 1060 - 848 615<br />
670 - - - - - - -<br />
710 - 2070 - 1190 - 952 690<br />
750 - - - - - - -<br />
800 - 2340 - 1346 - 1076 780<br />
850 - - - - - - -<br />
900 - 2640 - 1518 - 1214 880<br />
950 - - - - - - -<br />
1000 - 2910 - 1673 - 1339 970<br />
Fig. A4 : Rated operational power and currents (concluded)<br />
3.2 Resistive-type heating appliances and<br />
incandescent lamps (conventional or halogen)<br />
The current demand <strong>of</strong> a heating appliance or an incandescent lamp is easily<br />
obtained from the nominal power Pn quoted by the manufacturer (i.e. cos ϕ = 1)<br />
(see Fig. A5).<br />
Fig. A5 : Current demands <strong>of</strong> resistive heating and incandescent lighting (conventional or<br />
halogen) appliances<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Nominal Current demand (A)<br />
power 1-phase 1-phase 3-phase 3-phase<br />
(kW) 127 V 230 V 230 V 400 V<br />
0.1 0.79 0.43 0.25 0.14<br />
0.2 1.58 0.87 0.50 0.29<br />
0.5 3.94 2.17 1.26 0.72<br />
1 7.9 4.35 2.51 1.44<br />
1.5 11.8 6.52 3.77 2.17<br />
2 15.8 8.70 5.02 2.89<br />
2.5 19.7 10.9 6.28 3.61<br />
3 23.6 13 7.53 4.33<br />
3.5 27.6 15.2 8.72 5.05<br />
4 31.5 17.4 10 5.77<br />
4.5 35.4 19.6 11.3 6.5<br />
5 39.4 21.7 12.6 7.22<br />
6 47.2 26.1 15.1 8.66<br />
7 55.1 30.4 17.6 10.1<br />
8 63 34.8 20.1 11.5<br />
9 71 39.1 22.6 13<br />
10 79 43.5 25.1 14.4
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
(1) Ia in amps; U in volts. Pn is in watts. If Pn is in kW, then<br />
multiply the equation by 1,000<br />
(2) “Power-factor correction” is <strong>of</strong>ten referred to as<br />
“compensation” in discharge-lighting-tube terminology.<br />
Cos ϕ is approximately 0.95 (the zero values <strong>of</strong> V and I<br />
are almost in phase) but the power factor is 0.5 due to the<br />
impulsive form <strong>of</strong> the current, the peak <strong>of</strong> which occurs “late”<br />
in each half cycle<br />
3 Installed power loads -<br />
Characteristics<br />
The currents are given by:<br />
b 3-phase case: I a = Pn<br />
3 U<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
(1)<br />
(1)<br />
b 1-phase case: I a = Pn<br />
U<br />
where U is the voltage between the terminals <strong>of</strong> the equipment.<br />
where U is the voltage between the terminals <strong>of</strong> the equipment.<br />
For an incandescent lamp, the use <strong>of</strong> halogen gas allows a more concentrated light<br />
source. The light output is increased and the lifetime <strong>of</strong> the lamp is doubled.<br />
Note: At the instant <strong>of</strong> switching on, the cold filament gives rise to a very brief but<br />
intense peak <strong>of</strong> current.<br />
Fluorescent lamps and related equipment<br />
The power Pn (watts) indicated on the tube <strong>of</strong> a fluorescent lamp does not include<br />
the power dissipated in the ballast.<br />
The current is is given by: by:<br />
P +<br />
I a = ballast Pn<br />
U cos �<br />
If no power-loss value is indicated for the ballast, a figure <strong>of</strong> 25% <strong>of</strong> Pn may be used.<br />
Where U = the voltage applied to the lamp, complete with its related equipment.<br />
If no power-loss value is indicated for the ballast, a figure <strong>of</strong> 25% <strong>of</strong> Pn may be used.<br />
Standard tubular fluorescent lamps<br />
With (unless otherwise indicated):<br />
b cos ϕ = 0.6 with no power factor (PF) correction (2) capacitor<br />
b cos ϕ = 0.86 with PF correction (2) (single or twin tubes)<br />
b cos ϕ = 0.96 for electronic ballast.<br />
If no power-loss value is indicated for the ballast, a figure <strong>of</strong> 25% <strong>of</strong> Pn may be used.<br />
Figure A6 gives these values for different arrangements <strong>of</strong> ballast.<br />
Arrangement Tube power Current (A) at 230 V Tube<br />
<strong>of</strong> lamps, starters (W) (3) Magnetic ballast Electronic length<br />
and ballasts ballast (cm)<br />
Without PF With PF<br />
correction correction<br />
capacitor capacitor<br />
Single tube 18 0.20 0.14 0.10 60<br />
36 0.33 0.23 0.18 120<br />
58 0.50 0.36 0.28 150<br />
Twin tubes 2 x 18 0.28 0.18 60<br />
2 x 36 0.46 0.35 120<br />
2 x 58 0.72 0.52 150<br />
(3) Power in watts marked on tube<br />
Fig. A6 : Current demands and power consumption <strong>of</strong> commonly-dimensioned fluorescent<br />
lighting tubes (at 230 V-50 Hz)<br />
Compact fluorescent lamps<br />
Compact fluorescent lamps have the same characteristics <strong>of</strong> economy and long life<br />
as classical tubes. They are commonly used in public places which are permanently<br />
illuminated (for example: corridors, hallways, bars, etc.) and can be mounted in<br />
situations otherwise illuminated by incandescent lamps (see Fig. A7 next page).<br />
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A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
The power in watts indicated on the tube <strong>of</strong><br />
a discharge lamp does not include the power<br />
dissipated in the ballast.<br />
Fig. A8 : Current demands <strong>of</strong> discharge lamps<br />
3 Installed power loads -<br />
Characteristics<br />
Fig. A7 : Current demands and power consumption <strong>of</strong> compact fluorescent lamps (at 230 V - 50 Hz)<br />
Discharge lamps<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Type <strong>of</strong> lamp Lamp power Current at 230 V<br />
(W) (A)<br />
Separated 10 0.080<br />
ballast lamp 18 0.110<br />
26 0.150<br />
Integrated 8 0.075<br />
ballast lamp 11 0.095<br />
16 0.125<br />
21 0.170<br />
Figure A8 gives the current taken by a complete unit, including all associated<br />
ancillary equipment.<br />
These lamps depend on the luminous <strong>electrical</strong> discharge through a gas or vapour<br />
<strong>of</strong> a metallic compound, which is contained in a hermetically-sealed transparent<br />
envelope at a pre-determined pressure. These lamps have a long start-up time,<br />
during which the current Ia is greater than the nominal current In. Power and current<br />
demands are given for different types <strong>of</strong> lamp (typical average values which may<br />
differ slightly from one manufacturer to another).<br />
Type <strong>of</strong> Power Current In(A) Starting Luminous Average Utilization<br />
lamp (W) demand PF not PF Ia/In Period efficiency timelife <strong>of</strong><br />
(W) at corrected corrected (mins) (lumens lamp (h)<br />
230 V 400 V 230 V 400 V 230 V 400 V per watt)<br />
High-pressure sodium vapour lamps<br />
50 60 0.76 0.3 1.4 to 1.6 4 to 6 80 to 120 9000 b Lighting <strong>of</strong><br />
70 80 1 0.45 large halls<br />
100 115 1.2 0.65 b Outdoor spaces<br />
150 168 1.8 0.85 b Public lighting<br />
250 274 3 1.4<br />
400 431 4.4 2.2<br />
1000 1055 10.45 4.9<br />
Low-pressure sodium vapour lamps<br />
26 34.5 0.45 0.17 1.1 to 1.3 7 to 15 100 to 200 8000 b Lighting <strong>of</strong><br />
36 46.5 0.22 to 12000 autoroutes<br />
66 80.5 0.39 b Security lighting,<br />
91 105.5 0.49 station<br />
131 154 0.69 b Platform, storage<br />
areas<br />
Mercury vapour + metal halide (also called metal-iodide)<br />
70 80.5 1 0.40 1.7 3 to 5 70 to 90 6000 b Lighting <strong>of</strong> very<br />
150 172 1.80 0.88 6000 large areas by<br />
250 276 2.10 1.35 6000 projectors (for<br />
400 425 3.40 2.15 6000 example: sports<br />
1000 1046 8.25 5.30 6000 stadiums, etc.)<br />
2000 2092 2052 16.50 8.60 10.50 6 2000<br />
Mercury vapour + fluorescent substance (fluorescent bulb)<br />
50 57 0.6 0.30 1.7 to 2 3 to 6 40 to 60 8000 b Workshops<br />
80 90 0.8 0.45 to 12000 with very high<br />
125 141 1.15 0.70 ceilings (halls,<br />
250 268 2.15 1.35 hangars)<br />
400 421 3.25 2.15 b Outdoor lighting<br />
700 731 5.4 3.85 b Low light output (1)<br />
1000 1046 8.25 5.30<br />
2000 2140 2080 15 11 6.1<br />
(1) Replaced by sodium vapour lamps.<br />
Note: these lamps are sensitive to voltage dips. They extinguish if the voltage falls to less than 50% <strong>of</strong> their nominal voltage, and will<br />
not re-ignite before cooling for approximately 4 minutes.<br />
Note: Sodium vapour low-pressure lamps have a light-output efficiency which is superior to that <strong>of</strong> all other sources. However, use <strong>of</strong><br />
these lamps is restricted by the fact that the yellow-orange colour emitted makes colour recognition practically impossible.
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
The installed power is the sum <strong>of</strong> the nominal<br />
powers <strong>of</strong> all power consuming devices in the<br />
<strong>installation</strong>.<br />
This is not the power to be actually supplied in<br />
practice.<br />
The installed apparent power is commonly<br />
assumed to be the arithmetical sum <strong>of</strong> the kVA<br />
<strong>of</strong> individual loads. The maximum estimated<br />
kVA to be supplied however is not equal to the<br />
total installed kVA.<br />
(1) For greater precision, account must be taken <strong>of</strong> the factor<br />
<strong>of</strong> maximum utilization as explained below in 4.3<br />
4 Power loading <strong>of</strong> an <strong>installation</strong><br />
In order to <strong>design</strong> an <strong>installation</strong>, the actual maximum load demand likely to be<br />
imposed on the power-supply system must be assessed.<br />
To base the <strong>design</strong> simply on the arithmetic sum <strong>of</strong> all the loads existing in the<br />
<strong>installation</strong> would be extravagantly uneconomical, and bad engineering practice.<br />
The aim <strong>of</strong> this chapter is to show how some factors taking into account the diversity<br />
(non simultaneous operation <strong>of</strong> all appliances <strong>of</strong> a given group) and utilization<br />
(e.g. an electric motor is not generally operated at its full-load capability, etc.) <strong>of</strong><br />
all existing and projected loads can be assessed. The values given are based on<br />
experience and on records taken from actual <strong>installation</strong>s. In addition to providing<br />
basic <strong>installation</strong>-<strong>design</strong> data on individual circuits, the results will provide a<br />
global value for the <strong>installation</strong>, from which the requirements <strong>of</strong> a supply system<br />
(distribution network, MV/LV transformer, or generating set) can be specified.<br />
4.1 Installed power (kW)<br />
Most <strong>electrical</strong> appliances and equipments are marked to indicate their nominal<br />
power rating (Pn).<br />
The installed power is the sum <strong>of</strong> the nominal powers <strong>of</strong> all power-consuming<br />
devices in the <strong>installation</strong>. This is not the power to be actually supplied in practice.<br />
This is the case for electric motors, where the power rating refers to the output power<br />
at its driving shaft. The input power consumption will evidently be greater<br />
Fluorescent and discharge lamps associated with stabilizing ballasts, are other<br />
cases in which the nominal power indicated on the lamp is less than the power<br />
consumed by the lamp and its ballast.<br />
Methods <strong>of</strong> assessing the actual power consumption <strong>of</strong> motors and lighting<br />
appliances are given in Section 3 <strong>of</strong> this <strong>Chapter</strong>.<br />
The power demand (kW) is necessary to choose the rated power <strong>of</strong> a generating set<br />
or battery, and where the requirements <strong>of</strong> a prime mover have to be considered.<br />
For a power supply from a LV public-supply network, or through a MV/LV transformer,<br />
the significant quantity is the apparent power in kVA.<br />
4.2 Installed apparent power (kVA)<br />
The installed apparent power is commonly assumed to be the arithmetical sum <strong>of</strong><br />
the kVA <strong>of</strong> individual loads. The maximum estimated kVA to be supplied however is<br />
not equal to the total installed kVA.<br />
The apparent-power demand <strong>of</strong> a load (which might be a single appliance) is<br />
obtained from its nominal power rating (corrected if necessary, as noted above for<br />
motors, etc.) and the application <strong>of</strong> the following coefficients:<br />
η = the per-unit efficiency = output kW / input kW<br />
cos ϕ = the power factor = kW / kVA<br />
The apparent-power kVA demand <strong>of</strong> the load<br />
Pa = Pn /(η x cos ϕ)<br />
From this value, the full-load current Ia (A) (1) From this value, the full-load current taken by the load will be:<br />
3<br />
Pa x 10<br />
cb Ia =<br />
V<br />
for<br />
From<br />
single phase-to-neutral connected load<br />
for single this phase-to-neutral value, the full-load connected current load<br />
Pa x 103<br />
c b Ia =<br />
3 x U<br />
for<br />
for three-phase<br />
single phase-to-neutral<br />
balanced load<br />
connected<br />
where:<br />
load<br />
V = phase-to-neutral voltage (volts)<br />
U = phase-to-phase voltage (volts)<br />
It may be noted that, strictly speaking, the total kVA <strong>of</strong> apparent power is not the<br />
arithmetical sum <strong>of</strong> the calculated kVA ratings <strong>of</strong> individual loads (unless all loads are<br />
at the same power factor).<br />
It is common practice however, to make a simple arithmetical summation, the result<br />
<strong>of</strong> which will give a kVA value that exceeds the true value by an acceptable “<strong>design</strong><br />
margin”.<br />
When some or all <strong>of</strong> the load characteristics are not known, the values shown<br />
in Figure A9 next page may be used to give a very approximate estimate <strong>of</strong> VA<br />
demands (individual loads are generally too small to be expressed in kVA or kW).<br />
The estimates for lighting loads are based on floor areas <strong>of</strong> 500 m2 .<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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A16<br />
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A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
4 Power loading <strong>of</strong> an <strong>installation</strong><br />
Fluorescent lighting (corrected to cos ϕ = 0.86)<br />
Type <strong>of</strong> application Estimated (VA/m 2 ) Average lighting<br />
fluorescent tube level (lux = lm/m 2 )<br />
with industrial reflector (1)<br />
Roads and highways 7 150<br />
storage areas, intermittent work<br />
Heavy-duty works: fabrication and 14 300<br />
assembly <strong>of</strong> very large work pieces<br />
Day-to-day work: <strong>of</strong>fice work 24 500<br />
Fine work: drawing <strong>of</strong>fices 41 800<br />
high-precision assembly workshops<br />
Power circuits<br />
Type <strong>of</strong> application Estimated (VA/m 2 )<br />
Pumping station compressed air 3 to 6<br />
Ventilation <strong>of</strong> premises 23<br />
Electrical convection heaters:<br />
private houses 115 to 146<br />
flats and apartments 90<br />
Offices 25<br />
Dispatching workshop 50<br />
Assembly workshop 70<br />
Machine shop 300<br />
Painting workshop 350<br />
Heat-treatment plant 700<br />
(1) example: 65 W tube (ballast not included), flux 5,100 lumens (Im),<br />
luminous efficiency <strong>of</strong> the tube = 78.5 Im / W.<br />
Fig. A9 : Estimation <strong>of</strong> installed apparent power<br />
4.3 Estimation <strong>of</strong> actual maximum kVA demand<br />
All individual loads are not necessarily operating at full rated nominal power nor<br />
necessarily at the same time. Factors ku and ks allow the determination <strong>of</strong> the<br />
maximum power and apparent-power demands actually required to dimension the<br />
<strong>installation</strong>.<br />
Factor <strong>of</strong> maximum utilization (ku)<br />
In normal operating conditions the power consumption <strong>of</strong> a load is sometimes less<br />
than that indicated as its nominal power rating, a fairly common occurrence that<br />
justifies the application <strong>of</strong> an utilization factor (ku) in the estimation <strong>of</strong> realistic values.<br />
This factor must be applied to each individual load, with particular attention to<br />
electric motors, which are very rarely operated at full load.<br />
In an industrial <strong>installation</strong> this factor may be estimated on an average at 0.75 for<br />
motors.<br />
For incandescent-lighting loads, the factor always equals 1.<br />
For socket-outlet circuits, the factors depend entirely on the type <strong>of</strong> appliances being<br />
supplied from the sockets concerned.<br />
Factor <strong>of</strong> simultaneity (ks)<br />
It is a matter <strong>of</strong> common experience that the simultaneous operation <strong>of</strong> all installed<br />
loads <strong>of</strong> a given <strong>installation</strong> never occurs in practice, i.e. there is always some degree<br />
<strong>of</strong> diversity and this fact is taken into account for estimating purposes by the use <strong>of</strong> a<br />
simultaneity factor (ks).<br />
The factor ks is applied to each group <strong>of</strong> loads (e.g. being supplied from a distribution<br />
or sub-distribution board). The determination <strong>of</strong> these factors is the responsibility<br />
<strong>of</strong> the <strong>design</strong>er, since it requires a detailed knowledge <strong>of</strong> the <strong>installation</strong> and the<br />
conditions in which the individual circuits are to be exploited. For this reason, it is not<br />
possible to give precise values for general application.<br />
Factor <strong>of</strong> simultaneity for an apartment block<br />
Some typical values for this case are given in Figure A10 opposite page, and are<br />
applicable to domestic consumers supplied at 230/400 V (3-phase 4-wires). In the<br />
case <strong>of</strong> consumers using <strong>electrical</strong> heat-storage units for space heating, a factor <strong>of</strong><br />
0.8 is recommended, regardless <strong>of</strong> the number <strong>of</strong> consumers.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
4 Power loading <strong>of</strong> an <strong>installation</strong><br />
Example (see Fig. A11):<br />
5 storeys apartment building with 25 consumers, each having 6 kVA <strong>of</strong> installed load.<br />
The total installed load for the building is: 36 + 24 + 30 + 36 + 24 = 150 kVA<br />
The apparent-power supply required for the building is: 150 x 0.46 = 69 kVA<br />
From Figure A10, it is possible to determine the magnitude <strong>of</strong> currents in different<br />
sections <strong>of</strong> the common main feeder supplying all floors. For vertical rising mains<br />
fed at ground level, the cross-sectional area <strong>of</strong> the conductors can evidently be<br />
progressively reduced from the lower floors towards the upper floors.<br />
These changes <strong>of</strong> conductor size are conventionally spaced by at least 3-floor<br />
intervals.<br />
In the example, the current entering the rising main at ground level is:<br />
3<br />
150 x 0.46 x 10<br />
= 100 A<br />
400 3<br />
the current entering the third floor is:<br />
3<br />
(36 + 24) x 0.63 x 10<br />
= 55 A<br />
400 3<br />
4th<br />
floor<br />
3 rd<br />
floor<br />
2nd<br />
floor<br />
1 st<br />
floor<br />
ground<br />
floor<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Number <strong>of</strong> downstream Factor <strong>of</strong><br />
consumers simultaneity (ks)<br />
2 to 4 1<br />
5 to 9 0.78<br />
10 to 14 0.63<br />
15 to 19 0.53<br />
20 to 24 0.49<br />
25 to 29 0.46<br />
30 to 34 0.44<br />
35 to 39 0.42<br />
40 to 49 0.41<br />
50 and more 0.40<br />
Fig. A10 : Simultaneity factors in an apartment block<br />
6 consumers<br />
36 kVA<br />
4 consumers<br />
24 kVA<br />
5 consumers<br />
30 kVA<br />
6 consumers<br />
36 kVA<br />
4 consumers<br />
24 kVA<br />
0.78<br />
0.63<br />
0.53<br />
0.49<br />
0.46<br />
Fig. A11 : Application <strong>of</strong> the factor <strong>of</strong> simultaneity (ks) to an apartment block <strong>of</strong> 5 storeys<br />
A17<br />
© Schneider Electric - all rights reserved
A18<br />
© Schneider Electric - all rights reserved<br />
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
4 Power loading <strong>of</strong> an <strong>installation</strong><br />
Factor <strong>of</strong> simultaneity for distribution boards<br />
Figure A12 shows hypothetical values <strong>of</strong> ks for a distribution board supplying a<br />
number <strong>of</strong> circuits for which there is no indication <strong>of</strong> the manner in which the total<br />
load divides between them.<br />
If the circuits are mainly for lighting loads, it is prudent to adopt ks values close to<br />
unity.<br />
Fig. A12 : Factor <strong>of</strong> simultaneity for distribution boards (IEC 60439)<br />
Circuit function Factor <strong>of</strong> simultaneity (ks)<br />
Lighting 1<br />
Heating and air conditioning 1<br />
Socket-outlets 0.1 to 0.2 (1)<br />
Lifts and catering hoist (2) b For the most powerful<br />
motor 1<br />
b For the second most<br />
powerful motor 0.75<br />
b For all motors 0.60<br />
(1) In certain cases, notably in industrial <strong>installation</strong>s, this factor can be higher.<br />
(2) The current to take into consideration is equal to the nominal current <strong>of</strong> the motor,<br />
increased by a third <strong>of</strong> its starting current.<br />
Fig. A13 : Factor <strong>of</strong> simultaneity according to circuit function<br />
4.4 Example <strong>of</strong> application <strong>of</strong> factors ku and ks<br />
An example in the estimation <strong>of</strong> actual maximum kVA demands at all levels <strong>of</strong> an<br />
<strong>installation</strong>, from each load position to the point <strong>of</strong> supply is given Fig. A14 (opposite<br />
page).<br />
In this example, the total installed apparent power is 126.6 kVA, which corresponds<br />
to an actual (estimated) maximum value at the LV terminals <strong>of</strong> the MV/LV transformer<br />
<strong>of</strong> 65 kVA only.<br />
Note: in order to select cable sizes for the distribution circuits <strong>of</strong> an <strong>installation</strong>, the<br />
current I (in amps) through a circuit is determined from the equation:<br />
I = kVA x 10<br />
U<br />
3<br />
3<br />
where kVA is the actual maximum 3-phase apparent-power value shown on the<br />
diagram for the circuit concerned, and U is the phase to- phase voltage (in volts).<br />
4.5 Diversity factor<br />
The term diversity factor, as defined in IEC standards, is identical to the factor <strong>of</strong><br />
simultaneity (ks) used in this guide, as described in 4.3. In some English-speaking<br />
countries however (at the time <strong>of</strong> writing) diversity factor is the inverse <strong>of</strong> ks i.e. it is<br />
always u 1.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Number <strong>of</strong> Factor <strong>of</strong><br />
circuits simultaneity (ks)<br />
Assemblies entirely tested 0.9<br />
2 and 3<br />
4 and 5 0.8<br />
6 to 9 0.7<br />
10 and more 0.6<br />
Assemblies partially tested 1.0<br />
in every case choose<br />
Factor <strong>of</strong> simultaneity according to circuit function<br />
ks factors which may be used for circuits supplying commonly-occurring loads, are<br />
shown in Figure A13.
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
Workshop A Lathe no. 1 5 0.8<br />
Pedestaldrill<br />
30 fluorescent<br />
lamps<br />
Workshop B Compressor<br />
Oven<br />
no. 2<br />
no. 3<br />
no. 4<br />
no. 1<br />
no. 2<br />
5 socketoutlets<br />
10/16 A<br />
3 socketoutlets<br />
10/16 A<br />
10 fluorescent<br />
lamps<br />
Workshop C Ventilation no. 1<br />
no. 2<br />
no. 1<br />
no. 2<br />
5 socketoutlets<br />
10/16 A<br />
20 fluorescent<br />
lamps<br />
5<br />
5<br />
5<br />
2<br />
2<br />
18<br />
0.8<br />
0.8<br />
0.8<br />
0.8<br />
0.8<br />
1<br />
4 Power loading <strong>of</strong> an <strong>installation</strong><br />
Distribution<br />
box<br />
3 1 3<br />
1<br />
15<br />
10.6<br />
1<br />
2.5<br />
2.5<br />
15<br />
15<br />
18<br />
2<br />
1<br />
0.8<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
4<br />
4<br />
4<br />
4<br />
1.6<br />
1.6<br />
18<br />
12<br />
10.6<br />
1<br />
2.5<br />
2.5<br />
15<br />
15<br />
18<br />
2<br />
Fig A14 : An example in estimating the maximum predicted loading <strong>of</strong> an <strong>installation</strong> (the factor values used are for demonstration purposes only)<br />
0.75<br />
0.2<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
1<br />
0.4<br />
1<br />
Distribution<br />
box<br />
1<br />
0.28<br />
1<br />
Level 1 Level 2 Level 3<br />
Utilization Apparent Utilization Apparent Simultaneity Apparent Simultaneity Apparent Simultaneity Apparent<br />
power factor power factor power factor power factor power<br />
(Pa) max. demand demand demand demand<br />
kVA max. kVA kVA kVA kVA<br />
14.4<br />
3.6<br />
3<br />
Power<br />
circuit<br />
Socketoulets<br />
Lighting<br />
circuit<br />
Powver<br />
circuit<br />
Workshop A<br />
distribution<br />
box<br />
Power<br />
circuit<br />
12 Workshop B<br />
Socketoulets<br />
distribution<br />
4.3 box<br />
1<br />
35<br />
5<br />
2<br />
Lighting<br />
circuit<br />
Socketoulets<br />
Lighting<br />
circuit<br />
0.9<br />
0.9<br />
Workshop C<br />
distribution<br />
box<br />
0.9<br />
18.9<br />
15.6<br />
37.8<br />
4.6 Choice <strong>of</strong> transformer rating<br />
Main<br />
general<br />
distribution<br />
board<br />
MGDB<br />
0.9<br />
65<br />
LV / MV<br />
When an <strong>installation</strong> is to be supplied directly from a MV/LV transformer and<br />
the maximum apparent-power loading <strong>of</strong> the <strong>installation</strong> has been determined, a<br />
suitable rating for the transformer can be decided, taking into account the following<br />
considerations (see Fig. A15):<br />
b The possibility <strong>of</strong> improving the power factor <strong>of</strong> the <strong>installation</strong> (see chapter L)<br />
b Anticipated extensions to the <strong>installation</strong><br />
b Installation constraints (e.g. temperature)<br />
b Standard transformer ratings<br />
Apparent power In (A)<br />
kVA 237 V 410 V<br />
100 244 141<br />
160 390 225<br />
250 609 352<br />
315 767 444<br />
400 974 563<br />
500 1218 704<br />
630 1535 887<br />
800 1949 1127<br />
1000 2436 1408<br />
1250 3045 1760<br />
1600 3898 2253<br />
2000 4872 2816<br />
2500 6090 3520<br />
3150 7673 4436<br />
Fig. A15 : Standard apparent powers for MV/LV transformers and related nominal output currents<br />
A19<br />
© Schneider Electric - all rights reserved
A20<br />
© Schneider Electric - all rights reserved<br />
A - <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong><br />
4 Power loading <strong>of</strong> an <strong>installation</strong><br />
The nominal full-load current In on the LV side <strong>of</strong> a 3-phase transformer is given by:<br />
a x 10<br />
In<br />
3<br />
= P<br />
U 3<br />
where<br />
b Pa = kVA rating <strong>of</strong> the transformer<br />
b U = phase-to-phase voltage at no-load in volts (237 V or 410 V)<br />
b In is in amperes.<br />
For a single-phase transformer:<br />
a x 10<br />
In<br />
3<br />
= P<br />
V<br />
where<br />
where<br />
b V = voltage between LV terminals at no-load (in volts)<br />
Simplified equation for 400 V (3-phase load)<br />
b In = kVA x 1.4<br />
The IEC standard for power transformers is IEC 60076.<br />
4.7 Choice <strong>of</strong> power-supply sources<br />
The importance <strong>of</strong> maintaining a continuous supply raises the question <strong>of</strong> the use <strong>of</strong><br />
standby-power plant. The choice and characteristics <strong>of</strong> these alternative sources are<br />
part <strong>of</strong> the architecture selection, as described in chapter D.<br />
For the main source <strong>of</strong> supply the choice is generally between a connection to the<br />
MV or the LV network <strong>of</strong> the power-supply utility.<br />
In practice, connection to a MV source may be necessary where the load exceeds<br />
(or is planned eventually to exceed) a certain level - generally <strong>of</strong> the order <strong>of</strong><br />
250 kVA, or if the quality <strong>of</strong> service required is greater than that normally available<br />
from a LV network.<br />
Moreover, if the <strong>installation</strong> is likely to cause disturbance to neighbouring consumers,<br />
when connected to a LV network, the supply authorities may propose a MV service.<br />
Supplies at MV can have certain advantages: in fact, a MV consumer:<br />
b Is not disturbed by other consumers, which could be the case at LV<br />
b Is free to choose any type <strong>of</strong> LV earthing system<br />
b Has a wider choice <strong>of</strong> economic tariffs<br />
b Can accept very large increases in load<br />
It should be noted, however, that:<br />
b The consumer is the owner <strong>of</strong> the MV/LV substation and, in some countries,<br />
he must build and equip it at his own expense. The power utility can, in certain<br />
circumstances, participate in the investment, at the level <strong>of</strong> the MV line for example<br />
b A part <strong>of</strong> the connection costs can, for instance, <strong>of</strong>ten be recovered if a second<br />
consumer is connected to the MV line within a certain time following the original<br />
consumer’s own connection<br />
b The consumer has access only to the LV part <strong>of</strong> the <strong>installation</strong>, access to the<br />
MV part being reserved to the utility personnel (meter reading, operations, etc.).<br />
However, in certain countries, the MV protective circuit-breaker (or fused load-break<br />
switch) can be operated by the consumer<br />
b The type and location <strong>of</strong> the substation are agreed between the consumer and<br />
the utility<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
2<br />
3<br />
4<br />
5<br />
6<br />
<strong>Chapter</strong> B<br />
Connection to the MV utility<br />
distribution network<br />
Contents<br />
Supply <strong>of</strong> power at medium voltage B2<br />
1.1 Power supply characteristics <strong>of</strong> medium voltage B2<br />
utility distribution network<br />
1.2 Different MV service connections B11<br />
1.3 Some operational aspects <strong>of</strong> MV distribution networks B12<br />
Procedure for the establishment <strong>of</strong> a new substation B 4<br />
2.1 Preliminary informations B14<br />
2.2 Project studies B15<br />
2.3 Implementation B15<br />
2.4 Commissioning B15<br />
Protection aspect B 6<br />
3.1 Protection against electric shocks B16<br />
3.2 Protection <strong>of</strong> transformer and circuits B17<br />
3.3 Interlocks and conditioned operations B19<br />
The consumer substation with LV metering B22<br />
4.1 <strong>General</strong> B22<br />
4.2 Choice <strong>of</strong> MV switchgear B22<br />
4.3 Choice <strong>of</strong> MV switchgear panel for a transformer circuit B25<br />
4.4 Choice <strong>of</strong> MV/LV transformer B25<br />
4.5 Instructions for use <strong>of</strong> MV equipment B29<br />
The consumer substation with MV metering B32<br />
5.1 <strong>General</strong> B32<br />
5.2 Choice <strong>of</strong> panels B34<br />
5.3 Parallel operation <strong>of</strong> transformers B35<br />
Constitution <strong>of</strong> MV/LV distribution substations B37<br />
6.1 Different types <strong>of</strong> substation B37<br />
6.2 Indoor substation B37<br />
6.3 Outdoor substation B39<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
B2<br />
B - Connection to the MV public<br />
distribution network<br />
The main features which characterize a powersupply<br />
system include:<br />
b The nominal voltage and related insulation<br />
levels<br />
b The short-circuit current<br />
b The rated normal current <strong>of</strong> items <strong>of</strong> plant<br />
and equipment<br />
b The earthing system<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
The term "medium voltage" is commonly used for distribution systems with voltages<br />
above 1 kV and generally applied up to and including 52 kV (see IEC 601-01-28<br />
Standard).<br />
In this chapter, distribution networks which operate at voltages <strong>of</strong> 1,000 V or less<br />
are referred to as Low-Voltage systems, while systems <strong>of</strong> power distribution which<br />
require one stage <strong>of</strong> stepdown voltage transformation, in order to feed into low voltage<br />
networks, will be referred to as Medium- Voltage systems.<br />
For economic and technical reasons the nominal voltage <strong>of</strong> medium-voltage<br />
distribution systems, as defined above, seldom exceeds 35 kV.<br />
. Power supply characteristics <strong>of</strong> medium voltage<br />
utility distribution network<br />
Nominal voltage and related insulation levels<br />
The nominal voltage <strong>of</strong> a system or <strong>of</strong> an equipment is defined in IEC 60038 Standard<br />
as “the voltage by which a system or equipment is <strong>design</strong>ated and to which certain<br />
operating characteristics are referred”. Closely related to the nominal voltage is the<br />
“highest voltage for equipment” which concerns the level <strong>of</strong> insulation at normal<br />
working frequency, and to which other characteristics may be referred in relevant<br />
equipment recommendations.<br />
The “highest voltage for equipment” is defined in IEC 60038 Standard as:<br />
“the maximum value <strong>of</strong> voltage for which equipment may be used, that occurs under<br />
normal operating conditions at any time and at any point on the system. It excludes<br />
voltage transients, such as those due to system switching, and temporary voltage<br />
variations”.<br />
Notes:<br />
- The highest voltage for equipment is indicated for nominal system voltages<br />
higher than 1,000 V only. It is understood that, particularly for some categories<br />
<strong>of</strong> equipment, normal operation cannot be ensured up to this "highest voltage for<br />
equipment", having regard to voltage sensitive characteristics such as losses <strong>of</strong><br />
capacitors, magnetizing current <strong>of</strong> transformers, etc. In such cases, IEC standards<br />
specify the limit to which the normal operation <strong>of</strong> this equipment can be ensured.<br />
2- It is understood that the equipment to be used in systems having nominal voltage<br />
not exceeding 1,000 V should be specified with reference to the nominal system<br />
voltage only, both for operation and for insulation.<br />
3- The definition for “highest voltage for equipment” given in IEC 60038 Standard<br />
is identical to the definition given in IEC 62271-1 Standard for “rated voltage”.<br />
IEC 62271-1 Standard concerns switchgear for voltages exceeding 1,000 V.<br />
The following values <strong>of</strong> Figure B , taken from IEC 60038 Standard, list the<br />
most-commonly used standard levels <strong>of</strong> medium-voltage distribution, and relate<br />
the nominal voltages to corresponding standard values <strong>of</strong> “Highest Voltage for<br />
Equipment”.<br />
These systems are generally three-wire systems unless otherwise indicated. The<br />
values shown are voltages between phases.<br />
The values indicated in parentheses should be considered as non-preferred values.<br />
It is recommended that these values should not be used for new systems to be<br />
constructed in future.<br />
It is recommended that in any one country the ratio between two adjacent nominal<br />
voltages should be not less than two.<br />
Series I (for 50 Hz and 60 Hz networks)<br />
Nominal system voltage Highest voltage for equipement<br />
(kV) (kV)<br />
3.3 (1) 3 (1) 3.6 (1)<br />
6.6 (1) 6 (1) 7.2 (1)<br />
11 10 12<br />
- 15 17.5<br />
22 20 24<br />
33 (2) - 36 (2)<br />
- 35 (2) 40.5 (2)<br />
(1) These values should not be used for public distribution systems.<br />
(2) The unification <strong>of</strong> these values is under consideration.<br />
Fig. B1 : Relation between nominal system voltages and highest voltages for the equipment<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
(1) This means basically that List 1 generally applies to<br />
switchgear to be used on underground-cable systems while<br />
List 2 is chosen for switchgear to be used on overhead-line<br />
systems.<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
In order to ensure adequate protection <strong>of</strong> equipment against abnormally-medium<br />
short term power-frequency overvoltages, and transient overvoltages caused by<br />
lightning, switching, and system fault conditions, etc. all MV equipment must be<br />
specified to have appropriate rated insulation levels.<br />
A "rated insulation level" is a set <strong>of</strong> specified dielectric withstand values covering<br />
various operating conditions. For MV equipment, in addition to the "highest voltage<br />
for equipment", it includes lightning impulse withstand and short-duration power<br />
frequency withstand.<br />
Switchgear<br />
Figure B2 shown below, lists normal values <strong>of</strong> “withstand” voltage requirements<br />
from IEC 62271-1 Standard. The choice between List 1 and List 2 values <strong>of</strong> table<br />
B2 depends on the degree <strong>of</strong> exposure to lightning and switching overvoltages (1) ,<br />
the type <strong>of</strong> neutral earthing, and the type <strong>of</strong> overvoltage protection devices, etc. (for<br />
further guidance reference should be made to IEC 60071).<br />
Rated Rated lightning impulse withstand voltage Rated short-duration<br />
voltage (peak value) power-frequency<br />
U (r.m.s. withstand voltage<br />
value) List List 2 (r.m.s. value)<br />
To earth, Across the To earth, Across the To earth, Across the<br />
between isolating between isolating between isolating<br />
poles distance poles distance poles distance<br />
and across and across and across<br />
open open open<br />
switching switching switching<br />
device device device<br />
(kV) (kV) (kV) (kV) (kV) (kV) (kV)<br />
3.6 20 23 40 46 10 12<br />
7.2 40 46 60 70 20 23<br />
12 60 70 75 85 28 32<br />
17.5 75 85 95 110 38 45<br />
24 95 110 125 145 50 60<br />
36 145 165 170 195 70 80<br />
52 - - 250 290 95 110<br />
72.5 - - 325 375 140 160<br />
Note: The withstand voltage values “across the isolating distance” are valid only for<br />
the switching devices where the clearance between open contacts is <strong>design</strong>ed to meet<br />
requirements specified for disconnectors (isolators).<br />
Fig. B2 : Switchgear rated insulation levels<br />
It should be noted that, at the voltage levels in question, no switching overvoltage<br />
ratings are mentioned. This is because overvoltages due to switching transients are<br />
less severe at these voltage levels than those due to lightning.<br />
Transformers<br />
Figure B3 shown below have been extracted from IEC 60076-3.<br />
The significance <strong>of</strong> list 1 and list 2 is the same as that for the switchgear table, i.e.<br />
the choice depends on the degree <strong>of</strong> exposure to lightning, etc.<br />
Highest voltage Rated short duration Rated lightning impulse<br />
for equipment power frequency withstand voltage<br />
(r.m.s.) withstand voltage (peak)<br />
(r.m.s.) List List 2<br />
(kV) (kV) (kV) (kV)<br />
y 1.1 3 - -<br />
3.6 10 20 40<br />
7.2 20 40 60<br />
12 28 60 75<br />
17.5 38 75 95<br />
24 50 95 125<br />
36 70 145 170<br />
52 95 250<br />
72.5 140 325<br />
Fig. B3 : Transformers rated insulation levels<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B3<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
B4<br />
B - Connection to the MV public<br />
distribution network<br />
The national standards <strong>of</strong> any particular country<br />
are normally rationalized to include one or two<br />
levels only <strong>of</strong> voltage, current, and fault-levels,<br />
etc.<br />
A circuit-breaker (or fuse switch, over a limited<br />
voltage range) is the only form <strong>of</strong> switchgear<br />
capable <strong>of</strong> safely breaking all kinds <strong>of</strong> fault<br />
currents occurring on a power system.<br />
I p<br />
Current (I)<br />
22I'' k<br />
t min<br />
22I b<br />
IDC 22Ik Time (t)<br />
Fig. B5 : Graphic representation <strong>of</strong> short-circuit quantities as<br />
per IEC 60909<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
Other components<br />
It is evident that the insulation performance <strong>of</strong> other MV components associated<br />
with these major items, e.g. porcelain or glass insulators, MV cables, instrument<br />
transformers, etc. must be compatible with that <strong>of</strong> the switchgear and<br />
transformers noted above. Test schedules for these items are given in appropriate<br />
IEC publications.<br />
The national standards <strong>of</strong> any particular country are normally rationalized to include<br />
one or two levels only <strong>of</strong> voltage, current, and fault-levels, etc.<br />
<strong>General</strong> note:<br />
The IEC standards are intended for worldwide application and consequently<br />
embrace an extensive range <strong>of</strong> voltage and current levels.<br />
These reflect the diverse practices adopted in countries <strong>of</strong> different meteorologic,<br />
geographic and economic constraints.<br />
Short-circuit current<br />
Standard values <strong>of</strong> circuit-breaker short-circuit current-breaking capability are<br />
normally given in kilo-amps.<br />
These values refer to a 3-phase short-circuit condition, and are expressed as the<br />
average <strong>of</strong> the r.m.s. values <strong>of</strong> the AC component <strong>of</strong> current in each <strong>of</strong> the three<br />
phases.<br />
For circuit-breakers in the rated voltage ranges being considered in this chapter,<br />
Figure B4 gives standard short-circuit current-breaking ratings.<br />
kV 3.6 7.2 2 7.5 24 36 52<br />
kA 8 8 8 8 8 8 8<br />
(rms) 10 12.5 12.5 12.5 12.5 12.5 12.5<br />
16 16 16 16 16 16 20<br />
25 25 25 25 25 25<br />
40 40 40 40 40 40<br />
50<br />
Fig. B4 : Standard short-circuit current-breaking ratings<br />
Short-circuit current calculation<br />
The <strong>rules</strong> for calculating short-circuit currents in <strong>electrical</strong> <strong>installation</strong>s are presented<br />
in IEC standard 60909.<br />
The calculation <strong>of</strong> short-circuit currents at various points in a power system can<br />
quickly turn into an arduous task when the <strong>installation</strong> is complicated.<br />
The use <strong>of</strong> specialized s<strong>of</strong>tware accelerates calculations.<br />
This general standard, applicable for all radial and meshed power systems, 50 or<br />
60 Hz and up to 550 kV, is extremely accurate and conservative.<br />
It may be used to handle the different types <strong>of</strong> solid short-circuit (symmetrical or<br />
dissymmetrical) that can occur in an <strong>electrical</strong> <strong>installation</strong>:<br />
b Three-phase short-circuit (all three phases), generally the type producing the<br />
highest currents<br />
b Two-phase short-circuit (between two phases), currents lower than three-phase faults<br />
b Two-phase-to-earth short-circuit (between two phases and earth)<br />
b Phase-to-earth short-circuit (between a phase and earth), the most frequent type<br />
(80% <strong>of</strong> all cases).<br />
When a fault occurs, the transient short-circuit current is a function <strong>of</strong> time and<br />
comprises two components (see Fig. B5).<br />
b An AC component, decreasing to its steady-state value, caused by the various<br />
rotating machines and a function <strong>of</strong> the combination <strong>of</strong> their time constants<br />
b A DC component, decreasing to zero, caused by the initiation <strong>of</strong> the current and a<br />
function <strong>of</strong> the circuit impedances<br />
Practically speaking, one must define the short-circuit values that are useful in<br />
selecting system equipment and the protection system:<br />
b I’’ k: rms value <strong>of</strong> the initial symmetrical current<br />
b Ib: rms value <strong>of</strong> the symmetrical current interrupted by the switching device when<br />
the first pole opens at tmin (minimum delay)<br />
b Ik: rms value <strong>of</strong> the steady-state symmetrical current<br />
b Ip: maximum instantaneous value <strong>of</strong> the current at the first peak<br />
b IDC: DC value <strong>of</strong> the current<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
These currents are identified by subscripts 3, 2, 2E, 1, depending on the type <strong>of</strong><br />
short-circuit, respectively three-phase, two-phase clear <strong>of</strong> earth, two-phase-to-earth,<br />
phase-to-earth.<br />
The method, based on the Thevenin superposition theorem and decomposition into<br />
symmetrical components, consists in applying to the short-circuit point an equivalent<br />
source <strong>of</strong> voltage in view <strong>of</strong> determining the current. The calculation takes place in<br />
three steps.<br />
b Define the equivalent source <strong>of</strong> voltage applied to the fault point. It represents the<br />
voltage existing just before the fault and is the rated voltage multiplied by a factor<br />
taking into account source variations, transformer on-load tap changers and the<br />
subtransient behavior <strong>of</strong> the machines.<br />
b Calculate the impedances, as seen from the fault point, <strong>of</strong> each branch arriving at<br />
this point. For positive and negative-sequence systems, the calculation does not take<br />
into account line capacitances and the admittances <strong>of</strong> parallel, non-rotating loads.<br />
b Once the voltage and impedance values are defined, calculate the characteristic<br />
minimum and maximum values <strong>of</strong> the short-circuit currents.<br />
The various current values at the fault point are calculated using:<br />
b The equations provided<br />
b A summing law for the currents flowing in the branches connected to the node:<br />
v I’’ k (see Fig. B6 for I’’ k calculation, where voltage factor c is defined by the<br />
standard; geometric or algebraic summing)<br />
v I p = κ x 2 x I’’ k, where κ is less than 2, depending on the R/X ratio <strong>of</strong> the positive<br />
sequence impedance for the given branch; peak summing<br />
v I b = μ x q x I’’ k, where μ and q are less than 1, depending on the generators and<br />
motors, and the minimum current interruption delay; algebraic summing<br />
v I k = I’’ k, when the fault is far from the generator<br />
v I k = λ x I r, for a generator, where Ir is the rated generator current and λ is a factor<br />
depending on its saturation inductance; algebraic summing.<br />
Type <strong>of</strong> short-circuit I’’ k<br />
<strong>General</strong> situation Distant faults<br />
3-phase<br />
2-phase<br />
2-phase-to-earth<br />
Phase-to-earth + 0 + 1 0<br />
Fig. B6 : Short-circuit currents as per IEC 60909<br />
Characterization<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
c Un<br />
3 Z1 c Un<br />
Z1 + Z2<br />
c Un 3 Z2<br />
Z1 Z2 + Z2 Z0 + Z1 Z0<br />
c Un 3<br />
Z1<br />
+Z +Z 2 0<br />
c Un<br />
3 Z1 c Un<br />
2Z1<br />
c Un 3<br />
1 + Z 2Z0 c Un 3<br />
2 Z1 + Z0<br />
There are 2 types <strong>of</strong> system equipment, based on whether or not they react when a<br />
fault occurs.<br />
Passive equipment<br />
This category comprises all equipment which, due to its function, must have<br />
the capacity to transport both normal current and short-circuit current.<br />
This equipment includes cables, lines, busbars, disconnecting switches, switches,<br />
transformers, series reactances and capacitors, instrument transformers.<br />
For this equipment, the capacity to withstand a short-circuit without damage<br />
is defined in terms <strong>of</strong>:<br />
b Electrodynamic withstand (“peak withstand current”; value <strong>of</strong> the peak current<br />
expressed in kA), characterizing mechanical resistance to electrodynamic stress<br />
b Thermal withstand (“short time withstand current”; rms value expressed in kA<br />
for duration between 0,5 and 3 seconds, with a preferred value <strong>of</strong> 1 second),<br />
characterizing maximum permissible heat dissipation.<br />
B5<br />
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© Schneider Electric - all rights reserved<br />
B6<br />
B - Connection to the MV public<br />
distribution network<br />
Current (I)<br />
I AC<br />
I DC<br />
I AC : Peak <strong>of</strong> the periodic component<br />
I DC : Aperiodic component<br />
Time (t)<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
Active equipment<br />
This category comprises the equipment <strong>design</strong>ed to clear short-circuit currents, i.e.<br />
circuit-breakers and fuses. This property is expressed by the breaking capacity and,<br />
if required, the making capacity when a fault occurs.<br />
b Breaking capacity (see Fig. B7)<br />
This basic characteristic <strong>of</strong> a fault interrupting device is the maximum current (rms<br />
value expressed in kA) it is capable <strong>of</strong> breaking under the specific conditions defined<br />
by the standards; in the IEC 62271-100 standard, it refers to the rms value <strong>of</strong> the<br />
AC component <strong>of</strong> the short-circuit current. In some other standards, the rms value<br />
<strong>of</strong> the sum <strong>of</strong> the 2 components (AC and DC) is specified, in which case, it is the<br />
“asymmetrical current”.<br />
The breaking capacity depends on other factors such as:<br />
v Voltage<br />
v R/X ratio <strong>of</strong> the interrupted circuit<br />
v Power system natural frequency<br />
v Number <strong>of</strong> breaking operations at maximum current, for example the cycle:<br />
O - C/O - C/O (O = opening, C = closing)<br />
The breaking capacity is a relatively complicated characteristic to define and it<br />
therefore comes as no surprise that the same device can be assigned different<br />
breaking capacities depending on the standard by which it is defined.<br />
b Short-circuit making capacity<br />
In general, this characteristic is implicitly defined by the breaking capacity because a<br />
device should be able to close for a current that it can break.<br />
Sometimes, the making capacity needs to be higher, for example for circuit-breakers<br />
protecting generators.<br />
The making capacity is defined in terms <strong>of</strong> peak value (expressed in kA) because the<br />
first asymmetric peak is the most demanding from an electrodynamic point <strong>of</strong> view.<br />
For example, according to standard IEC 62271-100, a circuit-breaker used in a 50 Hz<br />
power system must be able to handle a peak making current equal to 2.5 times the<br />
rms breaking current (2.6 times for 60 Hz systems).<br />
Making capacity is also required for switches, and sometimes for disconnectors, even<br />
if these devices are not able to clear the fault.<br />
b Prospective short-circuit breaking current<br />
Some devices have the capacity to limit the fault current to be interrupted.<br />
Their breaking capacity is defined as the maximum prospective breaking current that<br />
would develop during a solid short-circuit across the upstream terminals <strong>of</strong> the device.<br />
Specific device characteristics<br />
The functions provided by various interrupting devices and their main constraints are<br />
presented in Figure B8.<br />
Device Isolation <strong>of</strong> Current switching Main constrains<br />
two active conditions<br />
networks Normal Fault<br />
Disconnector Yes No No Longitudinal input/output isolation<br />
Switch No Yes No Making and breaking <strong>of</strong> normal<br />
load current<br />
Short-circuit making capacity<br />
Contactor No Yes No Rated making and breaking<br />
capacities<br />
Maximum making and breaking<br />
capacities<br />
Duty and endurance<br />
characteristics<br />
Circuit-breaker No Yes Yes Short-circuit breaking capacity<br />
Short-circuit making capacity<br />
Fuse No No Yes Minimum short-circuit breaking<br />
capacity<br />
Maximum short-circuit breaking<br />
capacity<br />
Fig. B7 : Rated breaking current <strong>of</strong> a circuit-breaker subjected<br />
to a short-circuit as per IEC 60056 Fig. B8 : Functions provided by interrupting devices<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
The most common normal current rating for<br />
general-purpose MV distribution switchgear is<br />
400 A.<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
Rated normal current<br />
The rated normal current is defined as “the r.m.s. value <strong>of</strong> the current which can be<br />
carried continuously at rated frequency with a temperature rise not exceeding that<br />
specified by the relevant product standard”.<br />
The rated normal current requirements for switchgear are decided at the substation<br />
<strong>design</strong> stage.<br />
The most common normal current rating for general-purpose MV distribution<br />
switchgear is 400 A.<br />
In industrial areas and medium-load-density urban districts, circuits rated at 630 A<br />
are sometimes required, while at bulk-supply substations which feed into MV<br />
networks,<br />
800 A; 1,250 A; 1,600 A; 2,500 A and 4,000 A circuit-breakers are listed as standard<br />
ratings for incoming-transformer circuits, bus-section and bus-coupler CBs, etc.<br />
For MV/LV transformer with a normal primary current up to roughly 60 A,<br />
a MV switch-fuse combination can be used . For higher primary currents,<br />
switch-fuse combination usually does not have the required performances.<br />
There are no IEC-recommended rated current values for switch-fuse combinations.<br />
The actual rated current <strong>of</strong> a given combination, meaning a switchgear base<br />
and defined fuses, is provided by the manufacturer <strong>of</strong> the combination as a table<br />
"fuse reference / rated current". These values <strong>of</strong> the rated current are defined by<br />
considering parameters <strong>of</strong> the combination as:<br />
b Normal thermal current <strong>of</strong> the fuses<br />
b Necessary derating <strong>of</strong> the fuses, due to their usage within the enclosure.<br />
When combinations are used for protecting transformers, then further parameters<br />
are to be considered, as presented in Appendix A <strong>of</strong> the IEC 62271-105 and in the<br />
IEC 60787. They are mainly:<br />
b The normal MV current <strong>of</strong> the transformer<br />
b The possible need for overloading the transformer<br />
b The inrush magnetizing current<br />
b The MV short-circuit power<br />
b The tapping switch adjustment range.<br />
Manufacturers usually provide an application table "service voltage / transformer<br />
power / fuse reference" based on standard distribution network and transformer<br />
parameters, and such table should be used with care, if dealing with unusual<br />
<strong>installation</strong>s.<br />
In such a scheme, the load-break switch should be suitably fitted with a tripping<br />
device e.g. with a relay to be able to trip at low fault-current levels which must cover<br />
(by an appropriate margin) the rated minimum breaking current <strong>of</strong> the MV fuses. In<br />
this way, medium values <strong>of</strong> fault current which are beyond the breaking capability <strong>of</strong><br />
the load-break switch will be cleared by the fuses, while low fault-current values, that<br />
cannot be correctly cleared by the fuses, will be cleared by the tripped load-break<br />
switch.<br />
Influence <strong>of</strong> the ambient temperature and altitude on the rated current<br />
Normal-current ratings are assigned to all current-carrying <strong>electrical</strong> appliances,<br />
and upper limits are decided by the acceptable temperature rise caused by the<br />
I 2 R (watts) dissipated in the conductors, (where I = r.m.s. current in amperes and<br />
R = the resistance <strong>of</strong> the conductor in ohms), together with the heat produced<br />
by magnetic-hysteresis and eddy-current losses in motors, transformers, steel<br />
enclosures, etc. and dielectric losses in cables and capacitors, where appropriate.<br />
The temperature rise above the ambient temperature will depend mainly on the<br />
rate at which the heat is removed. For example, large currents can be passed<br />
through electric motor windings without causing them to overheat, simply because<br />
a cooling fan fixed to the shaft <strong>of</strong> the motor removes the heat at the same rate as it<br />
is produced, and so the temperature reaches a stable value below that which could<br />
damage the insulation and result in a burnt-out motor.<br />
The normal-current values recommended by IEC are based on ambientair<br />
temperatures common to temperate climates at altitudes not exceeding<br />
1,000 metres, so that items which depend on natural cooling by radiation and<br />
air-convection will overheat if operated at rated normal current in a tropical climate<br />
and/ or at altitudes exceeding 1,000 metres. In such cases, the equipment has to be<br />
derated, i.e. be assigned a lower value <strong>of</strong> normal current rating.<br />
The case <strong>of</strong> transformer is addressed in IEC 60076-2.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B7<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
B8<br />
B - Connection to the MV public<br />
distribution network<br />
Earth faults on medium-voltage systems<br />
can produce dangerous voltage levels on<br />
LV <strong>installation</strong>s. LV consumers (and substation<br />
operating personnel) can be safeguarded<br />
against this danger by:<br />
b Restricting the magnitude <strong>of</strong> MV earth-fault<br />
currents<br />
b Reducing the substation earthing resistance<br />
to the lowest possible value<br />
b Creating equipotential conditions at the<br />
substation and at the consumer’s <strong>installation</strong><br />
Fault<br />
HV LV<br />
I f<br />
I f<br />
R s<br />
Fig. B9 : Transferred potential<br />
V= I f R s<br />
Consumer<br />
(1) The others being unearthed. A particular case <strong>of</strong> earth-fault<br />
current limitation is by means <strong>of</strong> a Petersen coil.<br />
1<br />
2<br />
3<br />
N<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
Earthing systems<br />
Earthing and equipment-bonding earth connections require careful consideration,<br />
particularly regarding safety <strong>of</strong> the LV consumer during the occurrence <strong>of</strong> a shortcircuit<br />
to earth on the MV system.<br />
Earth electrodes<br />
In general, it is preferable, where physically possible, to separate the electrode<br />
provided for earthing exposed conductive parts <strong>of</strong> MV equipment from the electrode<br />
intended for earthing the LV neutral conductor. This is commonly practised in rural<br />
systems where the LV neutral-conductor earth electrode is installed at one or two<br />
spans <strong>of</strong> LV distribution line away from the substation.<br />
In most cases, the limited space available in urban substations precludes this<br />
practice, i.e. there is no possibility <strong>of</strong> separating a MV electrode sufficiently from<br />
a LV electrode to avoid the transference <strong>of</strong> (possibly dangerous) voltages to the<br />
LV system.<br />
Earth-fault current<br />
Earth-fault current levels at medium voltage are generally (unless deliberately<br />
restricted) comparable to those <strong>of</strong> a 3-phase short-circuit.<br />
Such currents passing through an earth electrode will raise its voltage to a medium<br />
value with respect to “remote earth” (the earth surrounding the electrode will be<br />
raised to a medium potential; “remote earth” is at zero potential).<br />
For example, 10,000 A <strong>of</strong> earth-fault current passing through an electrode with an<br />
(unusually low) resistance <strong>of</strong> 0.5 ohms will raise its voltage to 5,000 V.<br />
Providing that all exposed metal in the substation is “bonded” (connected together)<br />
and then connected to the earth electrode, and the electrode is in the form <strong>of</strong> (or is<br />
connected to) a grid <strong>of</strong> conductors under the floor <strong>of</strong> the substation, then there is no<br />
danger to personnel, since this arrangement forms an equipotential “cage” in which<br />
all conductive material, including personnel, is raised to the same potential.<br />
Transferred potential<br />
A danger exists however from the problem known as Transferred Potential. It will be<br />
seen in Figure B9 that the neutral point <strong>of</strong> the LV winding <strong>of</strong> the MV/LV transformer<br />
is also connected to the common substation earth electrode, so that the neutral<br />
conductor, the LV phase windings and all phase conductors are also raised to the<br />
electrode potential.<br />
Low-voltage distribution cables leaving the substation will transfer this potential to<br />
consumers <strong>installation</strong>s. It may be noted that there will be no LV insulation failure<br />
between phases or from phase to neutral since they are all at the same potential. It is<br />
probable, however, that the insulation between phase and earth <strong>of</strong> a cable or some<br />
part <strong>of</strong> an <strong>installation</strong> would fail.<br />
Solutions<br />
A first step in minimizing the obvious dangers <strong>of</strong> transferred potentials is to reduce<br />
the magnitude <strong>of</strong> MV earth-fault currents. This is commonly achieved by earthing the<br />
MV system through resistors or reactors at the star points <strong>of</strong> selected transformers (1) ,<br />
located at bulk-supply substations.<br />
A relatively medium transferred potential cannot be entirely avoided by this means,<br />
however, and so the following strategy has been adopted in some countries.<br />
The equipotential earthing <strong>installation</strong> at a consumer’s premises represents a remote<br />
earth, i.e. at zero potential. However, if this earthing <strong>installation</strong> were to be connected<br />
by a low-impedance conductor to the earth electrode at the substation, then the<br />
equipotential conditions existing in the substation would also exist at the consumer’s<br />
<strong>installation</strong>.<br />
Low-impedance interconnection<br />
This low-impedance interconnection is achieved simply by connecting the neutral<br />
conductor to the consumer’s equipotential <strong>installation</strong>, and the result is recognized as<br />
the TN earthing system (IEC 60364) as shown in diagram A <strong>of</strong> Figure B 0 next page.<br />
The TN system is generally associated with a Protective Multiple Earthing (PME)<br />
scheme, in which the neutral conductor is earthed at intervals along its length (every<br />
3rd or 4th pole on a LV overhead-line distributor) and at each consumer’s service<br />
position. It can be seen that the network <strong>of</strong> neutral conductors radiating from a<br />
substation, each <strong>of</strong> which is earthed at regular intervals, constitutes, together with<br />
the substation earthing, a very effective low-resistance earth electrode.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
Diagram Rs value<br />
A - TN-a<br />
MV LV<br />
C - TT-a<br />
MV LV<br />
E - TT-b<br />
MV LV<br />
R S<br />
R S<br />
R S<br />
R N<br />
1<br />
2<br />
3<br />
N<br />
1<br />
2<br />
3<br />
N<br />
1<br />
2<br />
3<br />
N<br />
B - IT-a<br />
MV LV<br />
R S<br />
D - IT-b<br />
MV LV<br />
F - IT-c<br />
MV LV<br />
In cases E and F the LV protective conductors (bonding exposed conductive parts) in the substation<br />
are earthed via the substation earth electrode, and it is therefore the substation LV equipment (only)<br />
that could be subjected to overvoltage.<br />
R S<br />
R S<br />
R N<br />
Fig. B10 : Maximum earthing resistance Rs at a MV/LV substation to ensure safety during a short-circuit to earth fault on the medium-voltage equipment for different<br />
earthing systems<br />
The combination <strong>of</strong> restricted earth-fault currents, equipotential <strong>installation</strong>s and<br />
low resistance substation earthing, results in greatly reduced levels <strong>of</strong> overvoltage<br />
and limited stressing <strong>of</strong> phase-to-earth insulation during the type <strong>of</strong> MV earth-fault<br />
situation described above.<br />
Limitation <strong>of</strong> the MV earth-fault current and earth resistance <strong>of</strong> the substation<br />
Another widely-used earthing system is shown in diagram C <strong>of</strong> Figure B10. It will be<br />
seen that in the TT system, the consumer’s earthing <strong>installation</strong> (being isolated from<br />
that <strong>of</strong> the substation) constitutes a remote earth.<br />
This means that, although the transferred potential will not stress the phase-to-phase<br />
insulation <strong>of</strong> the consumer’s equipment, the phase-to-earth insulation <strong>of</strong> all three<br />
phases will be subjected to overvoltage.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
1<br />
2<br />
3<br />
N<br />
1<br />
2<br />
3<br />
N<br />
1<br />
2<br />
3<br />
N<br />
Cases A and B<br />
No particular resistance value for Rs is imposed<br />
in these cases<br />
Cases C and D<br />
Uw - Uo<br />
Rs y<br />
Im<br />
Where<br />
Uw = the rated normal-frequency withstand<br />
voltage for low-voltage equipment at<br />
consumer <strong>installation</strong>s<br />
Uo = phase to neutral voltage at consumer's<br />
<strong>installation</strong>s<br />
Im = maximum value <strong>of</strong> MV earth-fault current<br />
Cases E and F<br />
Uws - U<br />
Rs y<br />
Im<br />
Where<br />
Uws = the normal-frequency withstand voltage<br />
for low-voltage equipments in the<br />
substation (since the exposed conductive<br />
parts <strong>of</strong> these equipments are earthed<br />
via Rs)<br />
U = phase to neutral voltage at the substation<br />
for the TT(s) system, but the phase-tophase<br />
voltage for the IT(s) system<br />
Im = maximum value <strong>of</strong> MV earth-fault current<br />
Notes:<br />
b For TN-a and IT-a, the MV and LV exposed conductive parts at the substation and those at the consumer’s <strong>installation</strong>s, together with the<br />
LV neutral point <strong>of</strong> the transformer, are all earthed via the substation electrode system.<br />
b For TT-a and IT-b, the MV and LV exposed conductive parts at the substation, together with the LV neutral point <strong>of</strong> the transformer are earthed via<br />
the substation electrode system.<br />
b For TT-b and IT-c, the LV neutral point <strong>of</strong> the transformer is separately earthed outside <strong>of</strong> the area <strong>of</strong> influence <strong>of</strong> the substation earth electrode.<br />
Uw and Uws are commonly given the (IEC 60364-4-44) value Uo + 1200 V, where Uo is the nominal phase-to-neutral voltage <strong>of</strong> the LV system<br />
concerned.<br />
B9<br />
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B 0<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
The strategy in this case, is to reduce the resistance <strong>of</strong> the substation earth<br />
electrode, such that the standard value <strong>of</strong> 5-second withstand-voltage-to-earth for<br />
LV equipment and appliances will not be exceeded.<br />
Practical values adopted by one national <strong>electrical</strong> power-supply authority, on its<br />
20 kV distribution systems, are as follows:<br />
b Maximum earth-fault current in the neutral connection on overhead line distribution<br />
systems, or mixed (O/H line and U/G cable) systems, is 300 A<br />
b Maximum earth-fault current in the neutral connection on underground systems is<br />
1,000 A<br />
The formula required to determine the maximum value <strong>of</strong> earthing resistance Rs at<br />
the<br />
the<br />
substation,<br />
substation,<br />
to<br />
to<br />
ensure<br />
ensure<br />
that<br />
that<br />
the<br />
the<br />
LV<br />
LV<br />
withstand<br />
withstand<br />
voltage<br />
voltage will<br />
will<br />
not<br />
not<br />
be<br />
be<br />
exceeded,<br />
exceeded,<br />
is:<br />
is:<br />
Uw �Uo<br />
Rs = in ohms (see cases C and D in Figure B10). C10).<br />
Im<br />
Where<br />
Where<br />
Uw = the lowest standard value (in volts) <strong>of</strong> short-term (5 s) withstand voltage for the<br />
consumer’s <strong>installation</strong> and appliances = Uo + 1200 V (IEC 60364-4-44)<br />
Uo = phase to neutral voltage (in volts) at the consumer’s LV service position<br />
Im = maximum earth-fault current on the MV system (in amps). This maximum earth<br />
fault current Im is the vectorial sum <strong>of</strong> maximum earth-fault current in the neutral<br />
connection and total unbalanced capacitive current <strong>of</strong> the network.<br />
A third form <strong>of</strong> system earthing referred to as the “IT” system in IEC 60364 is<br />
commonly used where continuity <strong>of</strong> supply is essential, e.g. in hospitals, continuousprocess<br />
manufacturing, etc. The principle depends on taking a supply from an<br />
unearthed source, usually a transformer, the secondary winding <strong>of</strong> which is<br />
unearthed, or earthed through a medium impedance (u1,000 ohms). In these cases,<br />
an insulation failure to earth in the low-voltage circuits supplied from the secondary<br />
windings will result in zero or negligible fault-current flow, which can be allowed to<br />
persist until it is convenient to shut-down the affected circuit to carry out repair work.<br />
Diagrams B, D and F (Figure B10)<br />
They show IT systems in which resistors (<strong>of</strong> approximately 1,000 ohms) are included<br />
in the neutral earthing lead.<br />
If however, these resistors were removed, so that the system is unearthed, the<br />
following notes apply.<br />
Diagram B (Figure B10)<br />
All phase wires and the neutral conductor are “floating” with respect to earth, to which<br />
they are “connected” via the (normally very medium) insulation resistances and (very<br />
small) capacitances between the live conductors and earthed metal (conduits, etc.).<br />
Assuming perfect insulation, all LV phase and neutral conductors will be raised by<br />
electrostatic induction to a potential approaching that <strong>of</strong> the equipotential conductors.<br />
In practice, it is more likely, because <strong>of</strong> the numerous earth-leakage paths <strong>of</strong> all live<br />
conductors in a number <strong>of</strong> <strong>installation</strong>s acting in parallel, that the system will behave<br />
similarly to the case where a neutral earthing resistor is present, i.e. all conductors<br />
will be raised to the potential <strong>of</strong> the substation earth.<br />
In these cases, the overvoltage stresses on the LV insulation are small or nonexistent.<br />
Diagrams D and F (Figure B10)<br />
In these cases, the medium potential <strong>of</strong> the substation (S/S) earthing system acts on<br />
the isolated LV phase and neutral conductors:<br />
b Through the capacitance between the LV windings <strong>of</strong> the transformer and the<br />
transformer tank<br />
b Through capacitance between the equipotential conductors in the S/S and the<br />
cores <strong>of</strong> LV distribution cables leaving the S/S<br />
b Through current leakage paths in the insulation, in each case.<br />
At positions outside the area <strong>of</strong> influence <strong>of</strong> the S/S earthing, system capacitances<br />
exist between the conductors and earth at zero potential (capacitances between<br />
cores are irrelevant - all cores being raised to the same potential).<br />
The result is essentially a capacitive voltage divider, where each “capacitor” is<br />
shunted by (leakage path) resistances.<br />
In general, LV cable and <strong>installation</strong> wiring capacitances to earth are much<br />
larger, and the insulation resistances to earth are much smaller than those <strong>of</strong> the<br />
corresponding parameters at the S/S, so that most <strong>of</strong> the voltage stresses appear at<br />
the substation between the transformer tank and the LV winding.<br />
The rise in potential at consumers’ <strong>installation</strong>s is not likely therefore to be a problem<br />
where the MV earth-fault current level is restricted as previously mentioned.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Overhead line<br />
Fig. B11 : Single-line service<br />
Underground cable<br />
ring main<br />
Fig. B12 : Ring-main service<br />
(1) Copper is cathodic to most other metals and therefore<br />
resists corrosion.<br />
(2) A ring main is a continuous distributor in the form <strong>of</strong> a<br />
closed loop, which originates and terminates on one set <strong>of</strong><br />
busbars. Each end <strong>of</strong> the loop is controlled by its own circuitbreaker.<br />
In order to improve operational flexibility the busbars<br />
are <strong>of</strong>ten divided into two sections by a normally closed bussection<br />
circuit-breaker, and each end <strong>of</strong> the ring is connected<br />
to a different section.<br />
An interconnector is a continuous untapped feeder connecting<br />
the busbars <strong>of</strong> two substations. Each end <strong>of</strong> the interconnector<br />
is usually controlled by a circuit beaker.<br />
An interconnector-distributor is an interconnector which<br />
supplies one or more distribution substations along its length.<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
All IT-earthed transformers, whether the neutral point is isolated or earthed through<br />
a medium impedance, are routinely provided with an overvoltage limiting device<br />
which will automatically connect the neutral point directly to earth if an overvoltage<br />
condition approaches the insulation-withstand level <strong>of</strong> the LV system.<br />
In addition to the possibilities mentioned above, several other ways in which these<br />
overvoltages can occur are described in Clause 3.1.<br />
This kind <strong>of</strong> earth-fault is very rare, and when does occur is quickly detected and<br />
cleared by the automatic tripping <strong>of</strong> a circuit-breaker in a properly <strong>design</strong>ed and<br />
constructed <strong>installation</strong>.<br />
Safety in situations <strong>of</strong> elevated potentials depends entirely on the provision <strong>of</strong><br />
properly arranged equipotential areas, the basis <strong>of</strong> which is generally in the form <strong>of</strong> a<br />
widemeshed grid <strong>of</strong> interconnected bare copper conductors connected to verticallydriven<br />
copper-clad (1) steel rods.<br />
The equipotential criterion to be respected is that which is mentioned in <strong>Chapter</strong> F<br />
dealing with protection against electric shock by indirect contact, namely: that the<br />
potential between any two exposed metal parts which can be touched simultaneously<br />
by any parts the body must never, under any circumstances, exceed 50 V in dry<br />
conditions, or 25 V in wet conditions.<br />
Special care should be taken at the boundaries <strong>of</strong> equipotential areas to avoid steep<br />
potential gradients on the surface <strong>of</strong> the ground which give rise to dangerous “step<br />
potentials”.<br />
This question is closely related to the safe earthing <strong>of</strong> boundary fences and is further<br />
discussed in Sub-clause 3.1.<br />
.2 Different MV service connections<br />
According to the type <strong>of</strong> medium-voltage network, the following supply arrangements<br />
are commonly adopted.<br />
Single-line service<br />
The substation is supplied by a single circuit tee-<strong>of</strong>f from a MV distributor (cable or<br />
line).<br />
In general, the MV service is connected into a panel containing a load-break/<br />
isolating switch-fuse combination and earthing switches, as shown in Figure B .<br />
In some countries a pole-mounted transformer with no MV switchgear or fuses<br />
(at the pole) constitutes the “substation”. This type <strong>of</strong> MV service is very common in<br />
rural areas.<br />
Protection and switching devices are remote from the transformer, and generally<br />
control a main overhead line, from which a number <strong>of</strong> these elementary service lines<br />
are tapped.<br />
Ring-main service<br />
Ring-main units (RMU) are normally connected to form a MV ring main (2) or<br />
interconnector-distributor (2) , such that the RMU busbars carry the full ring-main or<br />
interconnector current (see Fig. B 2).<br />
The RMU consists <strong>of</strong> three units, integrated to form a single assembly, viz:<br />
b 2 incoming units, each containing a load break/isolating switch and a circuit<br />
earthing switch<br />
b 1 outgoing and general protection unit, containing a load-break switch and<br />
MV fuses, or a combined load-break/fuse switch, or a circuit-breaker and isolating<br />
switch, together with a circuit-earthing switch in each case.<br />
All load-break switches and earthing switches are fully rated for short-circuit currentmaking<br />
duty.<br />
This arrangement provides the user with a two-source supply, thereby reducing<br />
considerably any interruption <strong>of</strong> service due to system faults or operations by the<br />
supply authority, etc.<br />
The main application for RMUs is in utility supply MV underground-cable networks in<br />
urban areas.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B<br />
© Schneider Electric - all rights reserved
B 2<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
Paralleled underground<br />
cable distributors<br />
Fig. B13 : Parallel feeders service<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
Parallel feeders service<br />
Where a MV supply connection to two lines or cables originating from the same<br />
busbar <strong>of</strong> a substation is possible, a similar MV switchboard to that <strong>of</strong> a RMU is<br />
commonly used (see Fig. B 3).<br />
The main operational difference between this arrangement and that <strong>of</strong> a RMU is that<br />
the two incoming panels are mutually interlocked, such that one incoming switch only<br />
can be closed at a time, i.e. its closure prevents the closure <strong>of</strong> the other.<br />
On the loss <strong>of</strong> power supply, the closed incoming switch must be opened and the<br />
(formerly open) switch can then be closed.<br />
The sequence may be carried out manually or automatically.<br />
This type <strong>of</strong> switchboard is used particularly in networks <strong>of</strong> medium-load density and<br />
in rapidly-expanding urban areas supplied by MV underground cable systems.<br />
.3 Some operational aspects <strong>of</strong> MV distribution<br />
networks<br />
Overhead lines<br />
Medium winds, ice formation, etc., can cause the conductors <strong>of</strong> overhead lines to<br />
touch each other, thereby causing a momentary (i.e. not permanent) short-circuit<br />
fault.<br />
Insulation failure due to broken ceramic or glass insulators, caused by air-borne<br />
debris; careless use <strong>of</strong> shot-guns, etc., or again, heavily polluted insulator surfaces,<br />
can result in a short-circuit to earth.<br />
Many <strong>of</strong> these faults are self-clearing. For example, in dry conditions, broken<br />
insulators can very <strong>of</strong>ten remain in service undetected, but are likely to flashover to<br />
earth (e.g. to a metal supporting structure) during a rainstorm. Moreover, polluted<br />
surfaces generally cause a flashover to earth only in damp conditions.<br />
The passage <strong>of</strong> fault current almost invariably takes the form <strong>of</strong> an electric arc, the<br />
intense heat <strong>of</strong> which dries the current path, and to some extent, re-establishes its<br />
insulating properties. In the meantime, protective devices have usually operated to<br />
clear the fault, i.e. fuses have blown or a circuit-breaker has tripped.<br />
Experience has shown that in the large majority <strong>of</strong> cases, restoration <strong>of</strong> supply by<br />
replacing fuses or by re-closing a circuit-breaker will be successful.<br />
For this reason it has been possible to considerably improve the continuity <strong>of</strong> service<br />
on MV overhead-line distribution networks by the application <strong>of</strong> automatic circuitbreaker<br />
reclosing schemes at the origin <strong>of</strong> the circuits concerned.<br />
These automatic schemes permit a number <strong>of</strong> reclosing operations if a first attempt<br />
fails, with adjustable time delays between successive attempts (to allow de-ionization<br />
<strong>of</strong> the air at the fault) before a final lock-out <strong>of</strong> the circuit-breaker occurs, after all<br />
(generally three) attempts fail.<br />
Other improvements in service continuity are achieved by the use <strong>of</strong> remotelycontrolled<br />
section switches and by automatic isolating switches which operate in<br />
conjunction with an auto-reclosing circuit-breaker.<br />
This last scheme is exemplified by the final sequence shown in Figure B 4 next<br />
page.<br />
The principle is as follows: if, after two reclosing attempts, the circuit-breaker trips,<br />
the fault is assumed to be permanent, then there are two possibilities:<br />
b The fault is on the section downstream the Automatic Line Switch, and while the<br />
feeder is dead the ALS opens to isolate this section <strong>of</strong> the network, before the third<br />
(and final) reclosing takes place,<br />
b The fault is on the section upstream the ALS and the circuit-breaker will make a<br />
third reclosing attempt and thus trip and lock out.<br />
While these measures have greatly improved the reliability <strong>of</strong> supplies from<br />
MV overhead line systems, the consumers must, where considered necessary, make<br />
their own arrangements to counter the effects <strong>of</strong> momentary interruptions to supply<br />
(between reclosures), for example:<br />
b Uninterruptible standby emergency power<br />
b Lighting that requires no cooling down before re-striking (“hot restrike”).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Centralized remote control, based on SCADA<br />
(Supervisory Control And Data Acquisition)<br />
systems and recent developments in IT<br />
(Information Technology) techniques, is<br />
becoming more and more common in countries<br />
in which the complexity <strong>of</strong> highly interconnected<br />
systems justifies the expenditure.<br />
Supply <strong>of</strong> power at medium<br />
voltage<br />
1- Cycle 1SR<br />
If O1 O2<br />
In Io 15 to 30 s<br />
fault<br />
0.3 s 0.4 s<br />
2 - Cycle 2SR<br />
a - Fault on main feeder<br />
If O1 O2<br />
In Io 15 to 30s<br />
fault<br />
b - Fault on section supplied through Automatic Line Switch<br />
If O1 O2<br />
SR1 O3<br />
In Io 15 to 30 s<br />
15 to 30 s<br />
Fault<br />
0.3 s 0.4 s<br />
0.3 s 0.4 s<br />
Fig. B14 : Automatic reclosing cycles <strong>of</strong> a circuit-breaker controlling a radial MV feeder<br />
Underground cable networks<br />
Faults on underground cable networks are sometimes the result <strong>of</strong> careless<br />
workmanship by cable jointers or by cable laying contractors, etc., but are more<br />
commonly due to damage from tools such as pick-axes, pneumatic drills and trench<br />
excavating machines, and so on, used by other utilities.<br />
Insulation failures sometimes occur in cable terminating boxes due to overvoltage,<br />
particularly at points in a MV system where an overhead line is connected to an<br />
underground cable. The overvoltage in such a case is generally <strong>of</strong> atmospheric<br />
origin, and electromagnetic-wave reflection effects at the joint box (where the natural<br />
impedance <strong>of</strong> the circuit changes abruptly) can result in overstressing <strong>of</strong> the cablebox<br />
insulation to the point <strong>of</strong> failure. Overvoltage protection devices, such as lightning<br />
arresters, are frequently installed at these locations.<br />
Faults occurring in cable networks are less frequent than those on overhead (O/H)<br />
line systems, but are almost invariably permanent faults, which require more time for<br />
localization and repair than those on O/H lines.<br />
Where a cable fault occurs on a ring, supply can be quickly restored to all consumers<br />
when the faulty section <strong>of</strong> cable has been determined.<br />
If, however, the fault occurs on a radial feeder, the delay in locating the fault and<br />
carrying out repair work can amount to several hours, and will affect all consumers<br />
downstream <strong>of</strong> the fault position. In any case, if supply continuity is essential on all,<br />
or part <strong>of</strong>, an <strong>installation</strong>, a standby source must be provided.<br />
Remote control <strong>of</strong> MV networks<br />
Remote control on MV feeders is useful to reduce outage durations in case <strong>of</strong> cable<br />
fault by providing an efficient and fast mean for loop configuration. This is achieved<br />
by motor operated switches implemented in some <strong>of</strong> the substations along the loop<br />
associated with relevant remote telecontrol units. Remote controled substation will<br />
always be reenergized through telecontroled operation when the other ones could<br />
have to wait for further manual operation.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
SR O3<br />
Permanent fault<br />
SR1 O3<br />
0.4 s<br />
0.4 s<br />
15 to 30 s<br />
SR2 O4<br />
Permanent fault<br />
0.45 s<br />
SR2<br />
Opening <strong>of</strong> ALS<br />
B 3<br />
© Schneider Electric - all rights reserved
B14<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
The consumer must provide certain data to the<br />
utility at the earliest stage <strong>of</strong> the project.<br />
2 Procedure for the establishment<br />
<strong>of</strong> a new substation<br />
Large consumers <strong>of</strong> electricity are invariably supplied at MV.<br />
On LV systems operating at 120/208 V (3-phase 4-wires), a load <strong>of</strong> 50 kVA might be<br />
considered to be “large”, while on a 240/415 V 3-phase system a “large” consumer<br />
could have a load in excess <strong>of</strong> 100 kVA. Both systems <strong>of</strong> LV distribution are common<br />
in many parts <strong>of</strong> the world.<br />
As a matter <strong>of</strong> interest, the IEC recommends a “world” standard <strong>of</strong> 230/400 V for<br />
3-phase 4-wire systems. This is a compromise level and will allow existing systems<br />
which operate at 220/380 V and at 240/415 V, or close to these values, to comply<br />
with the proposed standard simply by adjusting the <strong>of</strong>f-circuit tapping switches <strong>of</strong><br />
standard distribution transformers.<br />
The distance over which the energy has to be transmitted is a further factor in<br />
considering an MV or LV service. Services to small but isolated rural consumers are<br />
obvious examples.<br />
The decision <strong>of</strong> a MV or LV supply will depend on local circumstances and<br />
considerations such as those mentioned above, and will generally be imposed by the<br />
utility for the district concerned.<br />
When a decision to supply power at MV has been made, there are two widelyfollowed<br />
methods <strong>of</strong> proceeding:<br />
1 - The power-supplier constructs a standard substation close to the consumer’s<br />
premises, but the MV/LV transformer(s) is (are) located in transformer chamber(s)<br />
inside the premises, close to the load centre<br />
2 - The consumer constructs and equips his own substation on his own premises, to<br />
which the power supplier makes the MV connection<br />
In method no. 1 the power supplier owns the substation, the cable(s) to the<br />
transformer(s), the transformer(s) and the transformer chamber(s), to which he has<br />
unrestricted access.<br />
The transformer chamber(s) is (are) constructed by the consumer (to plans and<br />
regulations provided by the supplier) and include plinths, oil drains, fire walls and<br />
ceilings, ventilation, lighting, and earthing systems, all to be approved by the supply<br />
authority.<br />
The tariff structure will cover an agreed part <strong>of</strong> the expenditure required to provide<br />
the service.<br />
Whichever procedure is followed, the same principles apply in the conception and<br />
realization <strong>of</strong> the project. The following notes refer to procedure no. 2.<br />
2.1 Preliminary information<br />
Before any negotiations or discussions can be initiated with the supply authorities,<br />
the following basic elements must be established:<br />
Maximum anticipated power (kVA) demand<br />
Determination <strong>of</strong> this parameter is described in <strong>Chapter</strong> A, and must take into<br />
account the possibility <strong>of</strong> future additional load requirements. Factors to evaluate at<br />
this stage are:<br />
b The utilization factor (ku)<br />
b The simultaneity factor (ks)<br />
Layout plans and elevations showing location <strong>of</strong> proposed substation<br />
Plans should indicate clearly the means <strong>of</strong> access to the proposed substation, with<br />
dimensions <strong>of</strong> possible restrictions, e.g. entrances corridors and ceiling height,<br />
together with possible load (weight) bearing limits, and so on, keeping in mind that:<br />
b The power-supply personnel must have free and unrestricted access to the<br />
MV equipment in the substation at all times<br />
b Only qualified and authorized consumer’s personnel are allowed access to the<br />
substation<br />
b Some supply authorities or regulations require that the part <strong>of</strong> the <strong>installation</strong> operated<br />
by the authority is located in a separated room from the part operated by the customer.<br />
Degree <strong>of</strong> supply continuity required<br />
The consumer must estimate the consequences <strong>of</strong> a supply failure in terms <strong>of</strong> its<br />
duration:<br />
b Loss <strong>of</strong> production<br />
b Safety <strong>of</strong> personnel and equipment<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
The utility must give specific information to the<br />
prospective consumer.<br />
The utility must give <strong>of</strong>ficial approval <strong>of</strong> the<br />
equipment to be installed in the substation,<br />
and <strong>of</strong> proposed methods <strong>of</strong> <strong>installation</strong>.<br />
After testing and checking <strong>of</strong> the <strong>installation</strong> by<br />
an independent test authority, a certificate is<br />
granted which permits the substation to be put<br />
into service.<br />
2 Procedure for the establishment<br />
<strong>of</strong> a new substation<br />
2.2 Project studies<br />
From the information provided by the consumer, the power-supplier must indicate:<br />
The type <strong>of</strong> power supply proposed, and define:<br />
b The kind <strong>of</strong> power-supply system: overheadline or underground-cable network<br />
b Service connection details: single-line service, ring-main <strong>installation</strong>, or parallel<br />
feeders, etc.<br />
b Power (kVA) limit and fault current level<br />
The nominal voltage and rated voltage (Highest voltage for equipment)<br />
Existing or future, depending on the development <strong>of</strong> the system.<br />
Metering details which define:<br />
b The cost <strong>of</strong> connection to the power network<br />
b Tariff details (consumption and standing charges)<br />
2.3 Implementation<br />
Before any <strong>installation</strong> work is started, the <strong>of</strong>ficial agreement <strong>of</strong> the power-supplier<br />
must be obtained. The request for approval must include the following information,<br />
largely based on the preliminary exchanges noted above:<br />
b Location <strong>of</strong> the proposed substation<br />
b Single-line diagram <strong>of</strong> power circuits and connections, together with earthingcircuit<br />
proposals<br />
b Full details <strong>of</strong> <strong>electrical</strong> equipment to be installed, including performance<br />
characteristics<br />
b Layout <strong>of</strong> equipment and provision for metering components<br />
b Arrangements for power-factor improvement if required<br />
b Arrangements provided for emergency standby power plant (MV or LV) if eventually<br />
required<br />
2.4 Commissioning<br />
When required by the authority, commissioning tests must be successfully completed<br />
before authority is given to energize the <strong>installation</strong> from the power supply system.<br />
Even if no test is required by the authority it is better to do the following verification tests:<br />
b Measurement <strong>of</strong> earth-electrode resistances<br />
b Continuity <strong>of</strong> all equipotential earth-and safety bonding conductors<br />
b Inspection and functional testing <strong>of</strong> all MV components<br />
b Insulation checks <strong>of</strong> MV equipment<br />
b Dielectric strength test <strong>of</strong> transformer oil (and switchgear oil if appropriate), if<br />
applicable<br />
b Inspection and testing <strong>of</strong> the LV <strong>installation</strong> in the substation<br />
b Checks on all interlocks (mechanical key and <strong>electrical</strong>) and on all automatic<br />
sequences<br />
b Checks on correct protective-relay operation and settings<br />
It is also imperative to check that all equipment is provided, such that any properly<br />
executed operation can be carried out in complete safety. On receipt <strong>of</strong> the certificate<br />
<strong>of</strong> conformity (if required):<br />
b Personnel <strong>of</strong> the power-supply authority will energize the MV equipment and check<br />
for correct operation <strong>of</strong> the metering<br />
b The <strong>installation</strong> contractor is responsible for testing and connection <strong>of</strong> the<br />
LV <strong>installation</strong><br />
When finally the substation is operational:<br />
b The substation and all equipment belongs to the consumer<br />
b The power-supply authority has operational control over all MV switchgear in the<br />
substation, e.g. the two incoming load-break switches and the transformer MV switch<br />
(or CB) in the case <strong>of</strong> a RingMainUnit, together with all associated MV earthing switches<br />
b The power-supply personnel has unrestricted access to the MV equipment<br />
b The consumer has independent control <strong>of</strong> the MV switch (or CB) <strong>of</strong> the transformer(s)<br />
only, the consumer is responsible for the maintenance <strong>of</strong> all substation equipment,<br />
and must request the power-supply authority to isolate and earth the switchgear to<br />
allow maintenance work to proceed. The power supplier must issue a signed permitto-work<br />
to the consumers maintenance personnel, together with keys <strong>of</strong> locked-<strong>of</strong>f<br />
isolators, etc. at which the isolation has been carried out.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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B16<br />
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B - Connection to the MV public<br />
distribution network<br />
Protection against electric shocks and<br />
overvoltages is closely related to the<br />
achievement <strong>of</strong> efficient (low resistance)<br />
earthing and effective application <strong>of</strong> the<br />
principles <strong>of</strong> equipotential environments.<br />
3 Protection aspect<br />
The subject <strong>of</strong> protection in the <strong>electrical</strong> power industry is vast: it covers all aspects<br />
<strong>of</strong> safety for personnel, and protection against damage or destruction <strong>of</strong> property,<br />
plant, and equipment.<br />
These different aspects <strong>of</strong> protection can be broadly classified according to the<br />
following objectives:<br />
b Protection <strong>of</strong> personnel and animals against the dangers <strong>of</strong> overvoltages and<br />
electric shock, fire, explosions, and toxic gases, etc.<br />
b Protection <strong>of</strong> the plant, equipment and components <strong>of</strong> a power system against<br />
the stresses <strong>of</strong> short-circuit faults, atmospheric surges (lightning) and power-system<br />
instability (loss <strong>of</strong> synchronism) etc.<br />
b Protection <strong>of</strong> personnel and plant from the dangers <strong>of</strong> incorrect power-system<br />
operation, by the use <strong>of</strong> <strong>electrical</strong> and mechanical interlocking. All classes <strong>of</strong><br />
switchgear (including, for example, tap-position selector switches on transformers,<br />
and so on...) have well-defined operating limits. This means that the order in which<br />
the different kinds <strong>of</strong> switching device can be safely closed or opened is vitally<br />
important. Interlocking keys and analogous <strong>electrical</strong> control circuits are frequently<br />
used to ensure strict compliance with correct operating sequences.<br />
It is beyond the scope <strong>of</strong> a guide to describe in full technical detail the numerous<br />
schemes <strong>of</strong> protection available to power-systems engineers, but it is hoped that the<br />
following sections will prove to be useful through a discussion <strong>of</strong> general principles.<br />
While some <strong>of</strong> the protective devices mentioned are <strong>of</strong> universal application,<br />
descriptions generally will be confined to those in common use on MV and<br />
LV systems only, as defined in Sub-clause 1.1 <strong>of</strong> this <strong>Chapter</strong>.<br />
3.1 Protection against electric shocks<br />
Protective measures against electric shock are based on two common dangers:<br />
b Contact with an active conductor, i.e. which is live with respect to earth in normal<br />
circumstances. This is referred to as a “direct contact” hazard.<br />
b Contact with a conductive part <strong>of</strong> an apparatus which is normally dead, but which<br />
has become live due to insulation failure in the apparatus. This is referred to as an<br />
“indirect contact” hazard.<br />
It may be noted that a third type <strong>of</strong> shock hazard can exist in the proximity <strong>of</strong> MV or<br />
LV (or mixed) earth electrodes which are passing earth-fault currents. This hazard<br />
is due to potential gradients on the surface <strong>of</strong> the ground and is referred to as a<br />
“step-voltage” hazard; shock current enters one foot and leaves by the other foot, and<br />
is particular dangerous for four-legged animals. A variation <strong>of</strong> this danger, known as<br />
a “touch voltage” hazard can occur, for instance, when an earthed metallic part is<br />
situated in an area in which potential gradients exist.<br />
Touching the part would cause current to pass through the hand and both feet.<br />
Animals with a relatively long front-to-hind legs span are particularly sensitive to<br />
step-voltage hazards and cattle have been killed by the potential gradients caused by<br />
a low voltage (230/400 V) neutral earth electrode <strong>of</strong> insufficiently low resistance.<br />
Potential-gradient problems <strong>of</strong> the kind mentioned above are not normally<br />
encountered in <strong>electrical</strong> <strong>installation</strong>s <strong>of</strong> buildings, providing that equipotential<br />
conductors properly bond all exposed metal parts <strong>of</strong> equipment and all extraneous<br />
metal (i.e. not part <strong>of</strong> an <strong>electrical</strong> apparatus or the <strong>installation</strong> - for example<br />
structural steelwork, etc.) to the protective-earthing conductor.<br />
Direct-contact protection or basic protection<br />
The main form <strong>of</strong> protection against direct contact hazards is to contain all live parts<br />
in housings <strong>of</strong> insulating material or in metallic earthed housings, by placing out <strong>of</strong><br />
reach (behind insulated barriers or at the top <strong>of</strong> poles) or by means <strong>of</strong> obstacles.<br />
Where insulated live parts are housed in a metal envelope, for example transformers,<br />
electric motors and many domestic appliances, the metal envelope is connected to<br />
the <strong>installation</strong> protective earthing system.<br />
For MV switchgear, the IEC standard 62271-200 (Prefabricated Metal Enclosed<br />
switchgear and controlgear for voltages up to 52 kV) specifies a minimum Protection<br />
Index (IP coding) <strong>of</strong> IP2X which ensures the direct-contact protection. Furthermore,<br />
the metallic enclosure has to demonstrate an <strong>electrical</strong> continuity, then establishing<br />
a good segregation between inside and ouside <strong>of</strong> the enclosure. Proper grounding <strong>of</strong><br />
the enclosure further participates to the <strong>electrical</strong> protection <strong>of</strong> the operators under<br />
normal operating conditions.<br />
For LV appliances this is achieved through the third pin <strong>of</strong> a 3-pin plug and socket.<br />
Total or even partial failure <strong>of</strong> insulation to the metal, can raise the voltage <strong>of</strong> the<br />
envelope to a dangerous level (depending on the ratio <strong>of</strong> the resistance <strong>of</strong> the leakage<br />
path through the insulation, to the resistance from the metal envelope to earth).<br />
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B - Connection to the MV public<br />
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3 Protection aspect<br />
Indirect-contact protection or fault protection<br />
A person touching the metal envelope <strong>of</strong> an apparatus with a faulty insulation, as<br />
described above, is said to be making an indirect contact.<br />
An indirect contact is characterized by the fact that a current path to earth exists<br />
(through the protective earthing (PE) conductor) in parallel with the shock current<br />
through the person concerned.<br />
Case <strong>of</strong> fault on L.V. system<br />
Extensive tests have shown that, providing the potential <strong>of</strong> the metal envelope is not<br />
greater than 50 V with respect to earth, or to any conductive material within reaching<br />
distance, no danger exists.<br />
Indirect-contact hazard in the case <strong>of</strong> a MV fault<br />
If the insulation failure in an apparatus is between a MV conductor and the metal<br />
envelope, it is not generally possible to limit the rise <strong>of</strong> voltage <strong>of</strong> the envelope to<br />
50 V or less, simply by reducing the earthing resistance to a low value. The solution<br />
in this case is to create an equipotential situation, as described in Sub-clause 1.1<br />
“Earthing systems”.<br />
3.2 Protection <strong>of</strong> transformer and circuits<br />
<strong>General</strong><br />
The <strong>electrical</strong> equipment and circuits in a substation must be protected in order<br />
to avoid or to control damage due to abnormal currents and/or voltages. All<br />
equipment normally used in power system <strong>installation</strong>s have standardized short-time<br />
withstand ratings for overcurrent and overvoltage. The role <strong>of</strong> protective scheme is<br />
to ensure that this withstand limits can never be exceeded. In general, this means<br />
that fault conditions must be cleared as fast as possible without missing to ensure<br />
coordination between protective devices upstream and downstream the equipement<br />
to be protected. This means, when there is a fault in a network, generally several<br />
protective devices see the fault at the same time but only one must act.<br />
These devices may be:<br />
b Fuses which clear the faulty circuit directly or together with a mechanical tripping<br />
attachment, which opens an associated three-phase load-break switch<br />
b Relays which act indirectly on the circuit-breaker coil<br />
Transformer protection<br />
Stresses due to the supply network<br />
Some voltage surges can occur on the network such as :<br />
b Atmospheric voltage surges<br />
Atmospheric voltage surges are caused by a stroke <strong>of</strong> lightning falling on or near an<br />
overhead line.<br />
b Operating voltage surges<br />
A sudden change in the established operating conditions in an <strong>electrical</strong> network<br />
causes transient phenomena to occur. This is generally a high frequency or damped<br />
oscillation voltage surge wave.<br />
For both voltage surges, the overvoltage protection device generally used is a<br />
varistor (Zinc Oxide).<br />
In most cases, voltage surges protection has no action on switchgear.<br />
Stresses due to the load<br />
Overloading is frequently due to the coincidental demand <strong>of</strong> a number <strong>of</strong> small<br />
loads, or to an increase in the apparent power (kVA) demand <strong>of</strong> the <strong>installation</strong>,<br />
due to expansion in a factory, with consequent building extensions, and so on. Load<br />
increases raise the temperature <strong>of</strong> the wirings and <strong>of</strong> the insulation material. As<br />
a result, temperature increases involve a reduction <strong>of</strong> the equipment working life.<br />
Overload protection devices can be located on primary or secondary side <strong>of</strong> the<br />
transformer.<br />
The protection against overloading <strong>of</strong> a transformer is now provided by a digital relay<br />
which acts to trip the circuit-breaker on the secondary side <strong>of</strong> the transformer. Such<br />
relay, generally called thermal overload relay, artificially simulates the temperature,<br />
taking into account the time constant <strong>of</strong> the transformer. Some <strong>of</strong> them are able to<br />
take into account the effect <strong>of</strong> harmonic currents due to non linear loads (rectifiers,<br />
computer equipment, variable speed drives…).This type <strong>of</strong> relay is also able to<br />
predict the time before overload tripping and the waiting time after tripping. So, this<br />
information is very helpful to control load shedding operation.<br />
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B18<br />
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B - Connection to the MV public<br />
distribution network<br />
Fig. B15 : Transformer with conservator tank<br />
Fig. B16 : Totally filled transformer<br />
1<br />
2<br />
3<br />
Overcurrent relay<br />
E/F relay<br />
HV LV<br />
Fig. B17 : Protection against earth fault on the MV winding<br />
1<br />
2<br />
3<br />
N<br />
3 Protection aspect<br />
In addition, larger oil-immersed transformers frequently have thermostats with two<br />
settings, one for alarm purposes and the other for tripping.<br />
Dry-type transformers use heat sensors embedded in the hottest part <strong>of</strong> the windings<br />
insulation for alarm and tripping.<br />
Internal faults<br />
The protection <strong>of</strong> transformers by transformer-mounted devices, against the effects<br />
<strong>of</strong> internal faults, is provided on transformers which are fitted with airbreathing<br />
conservator tanks by the classical Buchholz mechanical relay (see Fig. B15). These<br />
relays can detect a slow accumulation <strong>of</strong> gases which results from the arcing <strong>of</strong><br />
incipient faults in the winding insulation or from the ingress <strong>of</strong> air due to an oil leak.<br />
This first level <strong>of</strong> detection generally gives an alarm, but if the condition deteriorates<br />
further, a second level <strong>of</strong> detection will trip the upstream circuit-breaker.<br />
An oil-surge detection feature <strong>of</strong> the Buchholz relay will trip the upstream circuitbreaker<br />
“instantaneously” if a surge <strong>of</strong> oil occurs in the pipe connecting the main tank<br />
with the conservator tank.<br />
Such a surge can only occur due to the displacement <strong>of</strong> oil caused by a rapidly<br />
formed bubble <strong>of</strong> gas, generated by an arc <strong>of</strong> short-circuit current in the oil.<br />
By specially <strong>design</strong>ing the cooling-oil radiator elements to perform a concerting action,<br />
“totally filled” types <strong>of</strong> transformer as large as 10 MVA are now currently available.<br />
Expansion <strong>of</strong> the oil is accommodated without an excessive rise in pressure by the<br />
“bellows” effect <strong>of</strong> the radiator elements. A full description <strong>of</strong> these transformers is<br />
given in Sub-clause 4.4 (see Fig. B16).<br />
Evidently the Buchholz devices mentioned above cannot be applied to this <strong>design</strong>; a<br />
modern counterpart has been developed however, which measures:<br />
b The accumulation <strong>of</strong> gas<br />
b Overpressure<br />
b Overtemperature<br />
The first two conditions trip the upstream circuit-breaker, and the third condition trips<br />
the downstream circuit-breaker <strong>of</strong> the transformer.<br />
Internal phase-to-phase short-circuit<br />
Internal phase-to-phase short-circuit must be detected and cleared by:<br />
b 3 fuses on the primary side <strong>of</strong> the tranformer or<br />
b An overcurrent relay that trips a circuit-breaker upstream <strong>of</strong> the transformer<br />
Internal phase-to-earth short-circuit<br />
This is the most common type <strong>of</strong> internal fault. It must be detected by an earth fault<br />
relay. Earth fault current can be calculated with the sum <strong>of</strong> the 3 primary phase<br />
currents (if 3 current transformers are used) or by a specific core current transformer.<br />
If a great sensitivity is needed, specific core current transformer will be prefered. In<br />
such a case, a two current transformers set is sufficient (see Fig. B17).<br />
Protection <strong>of</strong> circuits<br />
The protection <strong>of</strong> the circuits downstream <strong>of</strong> the transformer must comply with the<br />
IEC 60364 requirements.<br />
Discrimination between the protective devices upstream and<br />
downstream <strong>of</strong> the transformer<br />
The consumer-type substation with LV metering requires discriminative operation<br />
between the MV fuses or MV circuit-breaker and the LV circuit-breaker or fuses.<br />
The rating <strong>of</strong> the MV fuses will be chosen according to the characteristics <strong>of</strong> the<br />
transformer.<br />
The tripping characteristics <strong>of</strong> the LV circuit-breaker must be such that, for an<br />
overload or short-circuit condition downstream <strong>of</strong> its location, the breaker will trip<br />
sufficiently quickly to ensure that the MV fuses or the MV circuit-breaker will not be<br />
adversely affected by the passage <strong>of</strong> overcurrent through them.<br />
The tripping performance curves for MV fuses or MV circuit-breaker and LV circuitbreakers<br />
are given by graphs <strong>of</strong> time-to-operate against current passing through<br />
them. Both curves have the general inverse-time/current form (with an abrupt<br />
discontinuity in the CB curve at the current value above which “instantaneous”<br />
tripping occurs).<br />
These curves are shown typically in Figure B18.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Time<br />
D<br />
C<br />
A<br />
B<br />
Minimum pre-arcing<br />
time <strong>of</strong> MV fuse<br />
B/A u 1.35 at any<br />
moment in time<br />
D/C u 2 at any<br />
current value<br />
Circuit breaker<br />
tripping<br />
characteristic<br />
Current<br />
Fig. B18 : Discrimination between MV fuse operation and LV<br />
circuit-breaker tripping, for transformer protection<br />
U 1 MV LV U 2<br />
Fig. B19 : MV fuse and LV circuit-breaker configuration<br />
3 Protection aspect<br />
b In order to achieve discrimination:<br />
All parts <strong>of</strong> the fuse or MV circuit-breaker curve must be above and to the right <strong>of</strong> the<br />
CB curve.<br />
b In order to leave the fuses unaffected (i.e. undamaged):<br />
All parts <strong>of</strong> the minimum pre-arcing fuse curve must be located to the right <strong>of</strong> the CB<br />
curve by a factor <strong>of</strong> 1.35 or more (e.g. where, at time T, the CB curve passes through<br />
a point corresponding to 100 A, the fuse curve at the same time T must pass through<br />
a point corresponding to 135 A, or more, and so on...) and, all parts <strong>of</strong> the fuse curve<br />
must be above the CB curve by a factor <strong>of</strong> 2 or more (e.g. where, at a current level I<br />
the CB curve passes through a point corresponding to 1.5 seconds, the fuse curve<br />
at the same current level I must pass through a point corresponding to 3 seconds, or<br />
more, etc.).<br />
The factors 1.35 and 2 are based on standard maximum manufacturing tolerances<br />
for MV fuses and LV circuit-breakers.<br />
In order to compare the two curves, the MV currents must be converted to the<br />
equivalent LV currents, or vice-versa.<br />
Where a LV fuse-switch is used, similar separation <strong>of</strong> the characteristic curves <strong>of</strong> the<br />
MV and LV fuses must be respected.<br />
b In order to leave the MV circuit-breaker protection untripped:<br />
All parts <strong>of</strong> the minimum pre-arcing fuse curve must be located to the right <strong>of</strong> the<br />
CB curve by a factor <strong>of</strong> 1.35 or more (e.g. where, at time T, the LV CB curve passes<br />
through a point corresponding to 100 A, the MV CB curve at the same time T must<br />
pass through a point corresponding to 135 A, or more, and so on...) and, all parts <strong>of</strong><br />
the MV CB curve must be above the LV CB curve (time <strong>of</strong> LV CB curve must be less<br />
or equal than MV CB curves minus 0.3 s)<br />
The factors 1.35 and 0.3 s are based on standard maximum manufacturing<br />
tolerances for MV current transformers, MV protection relay and LV circuit-breakers.<br />
In order to compare the two curves, the MV currents must be converted to the<br />
equivalent LV currents, or vice-versa.<br />
Choice <strong>of</strong> protective device on the primary side <strong>of</strong> the<br />
transformer<br />
As explained before, for low reference current, the protection may be by fuses or by<br />
circuit-breaker.<br />
When the reference current is high, the protection will be achieved by circuit-breaker.<br />
Protection by circuit-breaker provides a more sensitive transformer protection<br />
compared with fuses. The implementation <strong>of</strong> additional protections (earth fault<br />
protection, thermal overload protection) is easier with circuit-breakers.<br />
3.3 Interlocks and conditioned operations<br />
Mechanical and <strong>electrical</strong> interlocks are included on mechanisms and in the control<br />
circuits <strong>of</strong> apparatus installed in substations, as a measure <strong>of</strong> protection against an<br />
incorrect sequence <strong>of</strong> manœuvres by operating personnel.<br />
Mechanical protection between functions located on separate equipment<br />
(e.g. switchboard and transformer) is provided by key-transfer interlocking.<br />
An interlocking scheme is intended to prevent any abnormal operational manœuvre.<br />
Some <strong>of</strong> such operations would expose operating personnel to danger, some others<br />
would only lead to an <strong>electrical</strong> incident.<br />
Basic interlocking<br />
Basic interlocking functions can be introduced in one given functionnal unit; some<br />
<strong>of</strong> these functions are made mandatory by the IEC 62271-200, for metal-enclosed<br />
MV switchgear, but some others are the result <strong>of</strong> a choice from the user.<br />
Considering access to a MV panel, it requires a certain number <strong>of</strong> operations<br />
which shall be carried out in a pre-determined order. It is necessary to carry out<br />
operations in the reverse order to restore the system to its former condition. Either<br />
proper procedures, or dedicated interlocks, can ensure that the required operations<br />
are performed in the right sequence. Then such accessible compartment will be<br />
classified as “accessible and interlocked” or “accessible by procedure”. Even for<br />
users with proper rigorous procedures, use <strong>of</strong> interlocks can provide a further help<br />
for safety <strong>of</strong> the operators.<br />
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B20<br />
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B - Connection to the MV public<br />
distribution network<br />
(1) If the earthing switch is on an incoming circuit, the<br />
associated isolating switches are those at both ends <strong>of</strong> the<br />
circuit, and these should be suitably interlocked. In such<br />
situation, the interlocking function becomes a multi-units key<br />
interlock.<br />
3 Protection aspect<br />
Key interlocking<br />
Beyond the interlocks available within a given functionnal unit (see also 4.2), the<br />
most widely-used form <strong>of</strong> locking/interlocking depends on the principle <strong>of</strong> key transfer.<br />
The principle is based on the possibility <strong>of</strong> freeing or trapping one or several keys,<br />
according to whether or not the required conditions are satisfied.<br />
These conditions can be combined in unique and obligatory sequences, thereby<br />
guaranteeing the safety <strong>of</strong> personnel and <strong>installation</strong> by the avoidance <strong>of</strong> an incorrect<br />
operational procedure.<br />
Non-observance <strong>of</strong> the correct sequence <strong>of</strong> operations in either case may have<br />
extremely serious consequences for the operating personnel, as well as for the<br />
equipment concerned.<br />
Note: It is important to provide for a scheme <strong>of</strong> interlocking in the basic <strong>design</strong> stage<br />
<strong>of</strong> planning a MV/LV substation. In this way, the apparatuses concerned will be<br />
equipped during manufacture in a coherent manner, with assured compatibility <strong>of</strong><br />
keys and locking devices.<br />
Service continuity<br />
For a given MV switchboard, the definition <strong>of</strong> the accessible compartments as well<br />
as their access conditions provide the basis <strong>of</strong> the “Loss <strong>of</strong> Service Continuity”<br />
classification defined in the standard IEC 62271-200. Use <strong>of</strong> interlocks or only proper<br />
procedure does not have any influence on the service continuity. Only the request for<br />
accessing a given part <strong>of</strong> the switchboard, under normal operation conditions, results<br />
in limiting conditions which can be more or less severe regarding the continuity <strong>of</strong> the<br />
<strong>electrical</strong> distribution process.<br />
Interlocks in substations<br />
In a MV/LV distribution substation which includes:<br />
b A single incoming MV panel or two incoming panels (from parallel feeders) or two<br />
incoming/outgoing ring-main panels<br />
b A transformer switchgear-and-protection panel, which can include a load-break/<br />
disconnecting switch with MV fuses and an earthing switch, or a circuit-breaker and<br />
line disconnecting switch together with an earthing switch<br />
b A transformer compartment<br />
Interlocks allow manœuvres and access to different panels in the following conditions:<br />
Basic interlocks, embedded in single functionnal units<br />
b Operation <strong>of</strong> the load-break/isolating switch<br />
v If the panel door is closed and the associated earthing switch is open<br />
b Operation <strong>of</strong> the line-disconnecting switch <strong>of</strong> the transformer switchgear - and<br />
- protection panel<br />
v If the door <strong>of</strong> the panel is closed, and<br />
v If the circuit-breaker is open, and the earthing switch(es) is (are) open<br />
b Closure <strong>of</strong> an earthing switch<br />
v If the associated isolating switch(es) is (are) open (1)<br />
b Access to an accessible compartment <strong>of</strong> each panel, if interlocks have been<br />
specified<br />
v If the isolating switch for the compartment is open and the earthing switch(es) for<br />
the compartment is (are) closed<br />
b Closure <strong>of</strong> the door <strong>of</strong> each accessible compartment, if interlocks have been<br />
specified<br />
v If the earthing switch(es) for the compartment is (are) closed<br />
Functional interlocks involving several functional units or separate equipment<br />
b Access to the terminals <strong>of</strong> a MV/LV transformer<br />
v If the tee-<strong>of</strong>f functional unit has its switch open and its earthing switch closed.<br />
According to the possibility <strong>of</strong> back-feed from the LV side, a condition on the LV main<br />
breaker can be necessary.<br />
Practical example<br />
In a consumer-type substation with LV metering, the interlocking scheme most<br />
commonly used is MV/LV/TR (high voltage/ low voltage/transformer).<br />
The aim <strong>of</strong> the interlocking is:<br />
b To prevent access to the transformer compartment if the earthing switch has not<br />
been previously closed<br />
b To prevent the closure <strong>of</strong> the earthing switch in a transformer switchgear-andprotection<br />
panel, if the LV circuit-breaker <strong>of</strong> the transformer has not been previously<br />
locked “open” or “withdrawn”<br />
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B - Connection to the MV public<br />
distribution network<br />
S<br />
MV switch and LV CB closed<br />
O<br />
S<br />
MV fuses accessible<br />
S<br />
Fig. B20 : Example <strong>of</strong> MV/LV/TR interlocking<br />
O<br />
O<br />
Transformer MV terminals accessible<br />
Legend<br />
Key absent<br />
Key free<br />
Key trapped<br />
Panel or door<br />
(1) Or may be provided with a common protective cover over<br />
the three terminals.<br />
S<br />
S<br />
S<br />
O<br />
3 Protection aspect<br />
Access to the MV or LV terminals <strong>of</strong> a transformer, (protected upstream by a<br />
MV switchgear-and-protection panel, containing a MV load-break / isolating<br />
switch, MV fuses, and a MV earthing switch) must comply with the strict procedure<br />
described below, and is illustrated by the diagrams <strong>of</strong> Figure B20.<br />
Note: The transformer in this example is provided with plug-in type MV terminal<br />
connectors which can only be removed by unlocking a retaining device common to<br />
all three phase connectors (1) .<br />
The MV load-break / disconnecting switch is mechanically linked with the<br />
MV earthing switch such that only one <strong>of</strong> the switches can be closed, i.e. closure<br />
<strong>of</strong> one switch automatically locks the closure <strong>of</strong> the other.<br />
Procedure for the isolation and earthing <strong>of</strong> the power transformer, and removal<br />
<strong>of</strong> the MV plug-type shrouded terminal connections (or protective cover)<br />
Initial conditions<br />
b MV load-break/disconnection switch and LV circuit-breaker are closed<br />
b MV earthing switch locked in the open position by key “O”<br />
b Key “O” is trapped in the LV circuit-breaker as long as that circuit-breaker is closed<br />
Step 1<br />
b Open LV CB and lock it open with key “O”<br />
b Key “O” is then released<br />
Step 2<br />
b Open the MV switch<br />
b Check that the “voltage presence” indicators extinguish when the MV switch is<br />
opened<br />
Step 3<br />
b Unlock the MV earthing switch with key “O” and close the earthing switch<br />
b Key “O” is now trapped<br />
Step 4<br />
The access panel to the MV fuses can now be removed (i.e. is released by closure <strong>of</strong><br />
the MV earthing switch). Key “S” is located in this panel, and is trapped when the MV<br />
switch is closed<br />
b Turn key “S” to lock the MV switch in the open position<br />
b Key “S” is now released<br />
Step 5<br />
Key “S” allows removal <strong>of</strong> the common locking device <strong>of</strong> the plug-type MV terminal<br />
connectors on the transformer or <strong>of</strong> the common protective cover over the terminals,<br />
as the case may be.<br />
In either case, exposure <strong>of</strong> one or more terminals will trap key “S” in the interlock.<br />
The result <strong>of</strong> the foregoing procedure is that:<br />
b The MV switch is locked in the open position by key “S”.<br />
Key “S” is trapped at the transformer terminals interlock as long as the terminals are<br />
exposed.<br />
b The MV earthing switch is in the closed position but not locked, i.e. may be opened<br />
or closed. When carrying out maintenance work, a padlock is generally used to lock<br />
the earthing switch in the closed position, the key <strong>of</strong> the padlock being held by the<br />
engineer supervizing the work.<br />
b The LV CB is locked open by key “O”, which is trapped by the closed MV earthing<br />
switch. The transformer is therefore safely isolated and earthed.<br />
It may be noted that the upstream terminal <strong>of</strong> the load-break disconnecting switch<br />
may remain live in the procedure described as the terminals in question are located<br />
in a separate non accessible compartment in the particular switchgear under<br />
discussion. Any other technical solution with exposed terminals in the accessed<br />
compartment would need further de-energisation and interlocks.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B21<br />
© Schneider Electric - all rights reserved
B22<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
(1) Polychlorinated biphenyl<br />
4 The consumer substation<br />
with LV metering<br />
4.1 <strong>General</strong><br />
A consumer substation with LV metering is an <strong>electrical</strong> <strong>installation</strong> connected to a<br />
utility supply system at a nominal voltage <strong>of</strong> 1 kV - 35 kV, and includes a single<br />
MV/LV transformer generally not exceeding 1,250 kVA.<br />
Functions<br />
The substation<br />
All component parts <strong>of</strong> the substation are located in one room, either in an existing<br />
building, or in the form <strong>of</strong> a prefabricated housing exterior to the building.<br />
Connection to the MV network<br />
Connection at MV can be:<br />
b Either by a single service cable or overhead line, or<br />
b Via two mechanically interlocked load-break switches with two service cables from<br />
duplicate supply feeders, or<br />
b Via two load-break switches <strong>of</strong> a ring-main unit<br />
The transformer<br />
Since the use <strong>of</strong> PCB (1) -filled transformers is prohibited in most countries,<br />
the preferred available technologies are:<br />
b Oil-immersed transformers for substations located outside premises<br />
b Dry-type, vacuum-cast-resin transformers for locations inside premises, e.g.<br />
multistoreyed buildings, buildings receiving the public, and so on...<br />
Metering<br />
Metering at low voltage allows the use <strong>of</strong> small metering transformers at modest cost.<br />
Most tariff structures take account <strong>of</strong> MV/LV transformer losses.<br />
LV <strong>installation</strong> circuits<br />
A low-voltage circuit-breaker, suitable for isolation duty and locking <strong>of</strong>f facilities, to:<br />
b Supply a distribution board<br />
b Protect the transformer against overloading and the downstream circuits against<br />
short-circuit faults.<br />
One-line diagrams<br />
The diagrams on the following page (see Fig. B21) represent the different methods<br />
<strong>of</strong> MV service connection, which may be one <strong>of</strong> four types:<br />
b Single-line service<br />
b Single-line service (equipped for extension to form a ring main)<br />
b Duplicate supply service<br />
b Ring main service<br />
4.2 Choice <strong>of</strong> MV switchgear<br />
Standards and specifications<br />
The switchgear and equipment described below are rated for 1 kV - 24 kV systems<br />
and comply with the following international standards:<br />
IEC 62271-1, 62271-200, 60265-1, 62271-102, 62271-100, 62271-105<br />
Local regulations can also require compliance with national standards as:<br />
b France: UTE<br />
b United Kingdom: BS<br />
b Germany: VDE<br />
b United States <strong>of</strong> America: ANSI<br />
Type <strong>of</strong> equipment<br />
In addition <strong>of</strong> Ring Main Units discussed in section 1.2, all kinds <strong>of</strong> switchgear<br />
arrangements are possible when using modular switchgear, and provisions for later<br />
extensions are easily realized.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Power supply<br />
system<br />
Service<br />
connection<br />
Supplier/consumer<br />
interface<br />
Single-line service<br />
Single-line service<br />
(equipped for extension<br />
to form a ring main)<br />
Duplicatesupply<br />
service<br />
Fig. B21 : Consumer substation with LV metering<br />
4 The consumer substation<br />
with LV metering<br />
MV protection and<br />
MV/LV transformation<br />
Protection<br />
Permitted if only one<br />
transformer and rated power<br />
low enough to accomodate<br />
the limitations <strong>of</strong> fuses and<br />
combinations<br />
Protection<br />
Permitted if only one<br />
transformer and rated power<br />
low enough to accomodate<br />
the limitations <strong>of</strong> fuses and<br />
combinations<br />
Transformer<br />
LV terminals<br />
LV metering<br />
and isolation<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
LV distribution<br />
and protection<br />
Downstream terminals<br />
<strong>of</strong> LV isolator<br />
Protection<br />
Protection<br />
+<br />
Auto-changeover<br />
switch<br />
Ring main<br />
service<br />
Protection<br />
Automatic<br />
LV standby<br />
source<br />
Always permitted<br />
B23<br />
© Schneider Electric - all rights reserved
B24<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
Fig. B22 : Metal enclosed MV load break disconnecting switch<br />
(1) Where MV fuses are used they are located in this<br />
compartment.<br />
4 The consumer substation<br />
with LV metering<br />
Operational safety <strong>of</strong> metal enclosed switchgear<br />
Description<br />
The following notes describe a “state-<strong>of</strong>-the art” load-break / disconnecting-switch<br />
panel (see Fig. B22) incorporating the most modern developments for ensuring:<br />
b Operational safety<br />
b Minimum space requirements<br />
b Extendibility and flexibility<br />
b Minimum maintenance requirements<br />
Each panel includes 3 compartments:<br />
b Switchgear: the load-break disconnecting switch is incorporated in an hermetically<br />
sealed (for life) molded epoxy-resin unit<br />
b Connections: by cable at terminals located on the molded switch unit<br />
b Busbars: modular, such that any number <strong>of</strong> panels may be assembled side-by-side<br />
to form a continuous switchboard, and for control and indication a low voltage cabinet<br />
which can accommodate automatic control and relaying equipment. An additional<br />
cabinet may be mounted above the existing one if further space is required.<br />
Cable connections are provided inside a cable-terminating compartment at the<br />
front <strong>of</strong> the unit, to which access is gained by removal <strong>of</strong> the front panel <strong>of</strong> the<br />
compartment.<br />
The units are connected <strong>electrical</strong>ly by means <strong>of</strong> prefabricated sections <strong>of</strong> busbars.<br />
Site erection is effected by following the assembly instructions.<br />
Operation <strong>of</strong> the switchgear is simplified by the grouping <strong>of</strong> all controls and<br />
indications on a control panel at the front <strong>of</strong> each unit.<br />
The technology <strong>of</strong> these switchgear units is essentially based on operational safety,<br />
ease <strong>of</strong> <strong>installation</strong> and low maintenance requirements.<br />
Switchgear internal safety measures<br />
b The load-break/disconnecting switch fully satisfies the requirement <strong>of</strong> “reliable<br />
position indicating device” as defined in IEC 62271-102 (disconnectors and earthing<br />
switches)<br />
b The functionnal unit incorporates the basic interlocks specified by the<br />
IEC 62271-200 (prefabricated metal enclosed switchgear and controlgear):<br />
v Closure <strong>of</strong> the switch is not possible unless the earth switch is open<br />
v Closure <strong>of</strong> the earthing switch is only possible if the load break/isolating switch is<br />
open<br />
b Access to the cable compartment, which is the only user-accessible compartment<br />
during operation, is secured by further interlocks:<br />
v Opening <strong>of</strong> the access panel to the cable terminations compartment (1) is only<br />
possible if the earthing switch is closed<br />
v The load-break/disconnecting switch is locked in the open position when the<br />
above-mentioned access panel is open. Opening <strong>of</strong> the earthing switch is then<br />
possible, for instance to allow a dielectric test on the cables.<br />
With such features, the switchboard can be operated with live busbars and cables,<br />
except for the unit where the access to cables is made. It complies then with the<br />
Loss <strong>of</strong> Service Continuity class LSB2A, as defined in the IEC 62271-200.<br />
Apart from the interlocks noted above, each switchgear panel includes:<br />
b Built-in padlocking facilities on the operation levers<br />
b 5 predrilled sets <strong>of</strong> fixing holes for possible future interlocking locks<br />
Operations<br />
b Operating handles, levers, etc. required for switching operations are grouped<br />
together on a clearly illustrated panel<br />
b All closing-operation levers are identical on all units (except those containing a<br />
circuit-breaker)<br />
b Operation <strong>of</strong> a closing lever requires very little effort<br />
b Opening or closing <strong>of</strong> a load-break/disconnecting switch can be by lever or by<br />
push-button for automatic switches<br />
b Conditions <strong>of</strong> switches (Open, Closed, Spring-charged), are clearly indicated<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
4 The consumer substation<br />
with LV metering<br />
4.3 Choice <strong>of</strong> MV switchgear panel for a transformer<br />
circuit<br />
Three types <strong>of</strong> MV switchgear panel are generally available:<br />
b Load-break switch and separate MV fuses in the panel<br />
b Load-break switch/MV fuses combination<br />
b Circuit-breaker<br />
Seven parameters influence the optimum choice:<br />
b The primary current <strong>of</strong> the transformer<br />
b The insulating medium <strong>of</strong> the transformer<br />
b The position <strong>of</strong> the substation with respect to the load centre<br />
b The kVA rating <strong>of</strong> the transformer<br />
b The distance from switchgear to the transformer<br />
b The use <strong>of</strong> separate protection relays (as opposed to direct-acting trip coils).<br />
Note: The fuses used in the load-break/switch fuses combination have striker-pins<br />
which ensure tripping <strong>of</strong> the 3-pole switch on the operation <strong>of</strong> one (or more) fuse(s).<br />
4.4 Choice <strong>of</strong> MV/LV transformer<br />
Characteristic parameters <strong>of</strong> a transformer<br />
A transformer is characterized in part by its <strong>electrical</strong> parameters, but also by its<br />
technology and its conditions <strong>of</strong> use.<br />
Electrical characteristics<br />
b Rated power (Pn): the conventional apparent-power in kVA on which other <strong>design</strong>parameter<br />
values and the construction <strong>of</strong> the transformer are based. Manufacturing<br />
tests and guarantees are referred to this rating<br />
b Frequency: for power distribution systems <strong>of</strong> the kind discussed in this guide, the<br />
frequency will be 50 Hz or 60 Hz<br />
b Rated primary and secondary voltages: For a primary winding capable <strong>of</strong> operating at<br />
more than one voltage level, a kVA rating corresponding to each level must be given.<br />
The secondary rated voltage is its open circuit value<br />
b Rated insulation levels are given by overvoltage-withstand test values at power<br />
frequency, and by high voltage impulse tests values which simulate lightning<br />
discharges. At the voltage levels discussed in this guide, overvoltages caused by<br />
MV switching operations are generally less severe than those due to lightning, so<br />
that no separate tests for switching-surge withstand capability are made<br />
b Off-circuit tap-selector switch generally allows a choice <strong>of</strong> up to ± 2.5% and ± 5%<br />
level about the rated voltage <strong>of</strong> the highest voltage winding. The transformer must be<br />
de-energized before this switch is operated<br />
b Winding configurations are indicated in diagrammatic form by standard symbols for<br />
star, delta and inter-connected-star windings; (and combinations <strong>of</strong> these for special<br />
duty, e.g. six-or twelve-phase rectifier transformers, etc.) and in an IEC-recommended<br />
alphanumeric code. This code is read from left-to-right, the first letter refers to the<br />
highest voltage winding, the second letter to the next highest, and so on:<br />
v Capital letters refer to the highest voltage winding<br />
D = delta<br />
Y = star<br />
Z = interconnected-star (or zigzag)<br />
N = neutral connection brought out to a terminal<br />
v Lower-case letters are used for tertiary and secondary windings<br />
d = delta<br />
y = star<br />
z = interconnected-star (or zigzag)<br />
n = neutral connection brought out to a terminal<br />
v A number from 0 to 11, corresponding to those, on a clock dial (“0” is used instead<br />
<strong>of</strong> “12”) follows any pair <strong>of</strong> letters to indicate the phase change (if any) which occurs<br />
during the transformation.<br />
A very common winding configuration used for distribution transformers is that<br />
<strong>of</strong> a Dyn 11 transformer, which has a delta MV winding with a star-connected<br />
secondary winding the neutral point <strong>of</strong> which is brought out to a terminal. The phase<br />
change through the transformer is +30 degrees, i.e. phase 1 secondary voltage is<br />
at “11 o’clock” when phase 1 <strong>of</strong> the primary voltage is at “12 o’clock”, as shown in<br />
Figure B31 page B34. All combinations <strong>of</strong> delta, star and zigzag windings produce a<br />
phase change which (if not zero) is either 30 degrees or a multiple <strong>of</strong> 30 degrees.<br />
IEC 60076-4 describes the “clock code” in detail.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B25<br />
© Schneider Electric - all rights reserved
B26<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
Fig. B23 : Dry-type transformer<br />
4 The consumer substation<br />
with LV metering<br />
Characteristics related to the technology and utilization <strong>of</strong> the transformer<br />
This list is not exhaustive:<br />
b Choice <strong>of</strong> technology<br />
The insulating medium is:<br />
v Liquid (mineral oil) or<br />
v Solid (epoxy resin and air)<br />
b For indoor or outdoor <strong>installation</strong><br />
b Altitude (
B - Connection to the MV public<br />
distribution network<br />
Fig. B24 : Hermetically-sealed totally-filled tank<br />
Fig. B25 : Air-breathing conservator-type tank at atmosphere<br />
pressure<br />
4 The consumer substation<br />
with LV metering<br />
There are two ways in which this pressure limitation is commonly achieved:<br />
b Hermetically-sealed totally-filled tank (up to 10 MVA at the present time)<br />
Developed by a leading French manufacturer in 1963, this method was adopted by the<br />
national utility in 1972, and is now in world-wide service (see Fig. B24).<br />
Expansion <strong>of</strong> the liquid is compensated by the elastic deformation <strong>of</strong> the oil-cooling<br />
passages attached to the tank.<br />
The “total-fill” technique has many important advantages over other methods:<br />
v Oxydation <strong>of</strong> the dielectric liquid (with atmospheric oxygen) is entirely precluded<br />
v No need for an air-drying device, and so no consequent maintenance (inspection<br />
and changing <strong>of</strong> saturated dessicant)<br />
v No need for dielectric-strength test <strong>of</strong> the liquid for at least 10 years<br />
v Simplified protection against internal faults by means <strong>of</strong> a DGPT device is possible<br />
v Simplicity <strong>of</strong> <strong>installation</strong>: lighter and lower pr<strong>of</strong>ile (than tanks with a conservator)<br />
and access to the MV and LV terminals is unobstructed<br />
v Immediate detection <strong>of</strong> (even small) oil leaks; water cannot enter the tank<br />
b Air-breathing conservator-type tank at atmospheric pressure<br />
Expansion <strong>of</strong> the insulating liquid is taken up by a change in the level <strong>of</strong> liquid in<br />
an expansion (conservator) tank, mounted above the transformer main tank, as<br />
shown in Figure B25. The space above the liquid in the conservator may be filled<br />
with air which is drawn in when the level <strong>of</strong> liquid falls, and is partially expelled<br />
when the level rises. When the air is drawn in from the surrounding atmosphere it is<br />
admitted through an oil seal, before passing through a dessicating device (generally<br />
containing silica-gel crystals) before entering the conservator. In some <strong>design</strong>s <strong>of</strong><br />
larger transformers the space above the oil is occupied by an impermeable air bag<br />
so that the insulation liquid is never in contact with the atmosphere. The air enters<br />
and exits from the deformable bag through an oil seal and dessicator, as previously<br />
described. A conservator expansion tank is obligatory for transformers rated above<br />
10 MVA (which is presently the upper limit for “total-fill” type transformers).<br />
Choice <strong>of</strong> technology<br />
As discussed above, the choice <strong>of</strong> transformer is between liquid-filled or dry type.<br />
For ratings up to 10 MVA, totally-filled units are available as an alternative to<br />
conservator-type transformers.<br />
A choice depends on a number <strong>of</strong> considerations, including:<br />
b Safety <strong>of</strong> persons in proximity to the transformer. Local regulations and <strong>of</strong>ficial<br />
recommendations may have to be respected<br />
b Economic considerations, taking account <strong>of</strong> the relative advantages <strong>of</strong> each technique<br />
The regulations affecting the choice are:<br />
b Dry-type transformer:<br />
v In some countries a dry-type transformer is obligatory in high apartment blocks<br />
v Dry-type transformers impose no constraints in other situations<br />
b Transformers with liquid insulation:<br />
v This type <strong>of</strong> transformer is generally forbidden in high apartment blocks<br />
v For different kinds <strong>of</strong> insulation liquids, <strong>installation</strong> restrictions, or minimum<br />
protection against fire risk, vary according to the class <strong>of</strong> insulation used<br />
v Some countries in which the use <strong>of</strong> liquid dielectrics is highly developed, classify<br />
the several categories <strong>of</strong> liquid according to their fire performance. This latter is<br />
assessed according to two criteria: the flash-point temperature, and the minimum<br />
calorific power. The principal categories are shown in Figure B26 in which a<br />
classification code is used for convenience.<br />
As an example, French standard defines the conditions for the <strong>installation</strong> <strong>of</strong> liquidfilled<br />
transformers. No equivalent IEC standard has yet been established.<br />
The French standard is aimed at ensuring the safety <strong>of</strong> persons and property and<br />
recommends, notably, the minimum measures to be taken against the risk <strong>of</strong> fire.<br />
Code Dielectric fluid Flash-point Minimum calorific power<br />
(°C) (MJ/kg)<br />
O1 Mineral oil < 300 -<br />
K1 High-density hydrocarbons > 300 48<br />
K2 Esters > 300 34 - 37<br />
K3 Silicones > 300 27 - 28<br />
L3 Insulating halogen liquids - 12<br />
Fig. B26 : Categories <strong>of</strong> dielectric fluids<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B27<br />
© Schneider Electric - all rights reserved
B28<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
4 The consumer substation<br />
with LV metering<br />
The main precautions to observe are indicated in Figure B27.<br />
b For liquid dielectrics <strong>of</strong> class L3 there are no special measures to be taken<br />
b For dielectrics <strong>of</strong> classes O1 and K1 the measures indicated are applicable only if<br />
there are more than 25 litres <strong>of</strong> dielectric liquid in the transformer<br />
b For dielectrics <strong>of</strong> classes K2 and K3 the measures indicated are applicable only if<br />
there are more than 50 litres <strong>of</strong> dielectric liquid in the transformer.<br />
Class No. <strong>of</strong> Locations<br />
<strong>of</strong> litres above Chamber or enclosed area reserved to qualified Reserved to trained personnel Other chambers<br />
dielectric which and authorized personnel, and separated from any and isolated from work areas or locations (2)<br />
fluid measures other building by a distance D by fire-pro<strong>of</strong> walls (2 hours rating)<br />
must be D > 8 m 4 m < D < 8 m D < 4 m (1) in the direc- No openings With opening(s)<br />
taken tion <strong>of</strong> occupied areas<br />
O1 25 No special Interposition <strong>of</strong> Fire-pro<strong>of</strong> wall Measures Measures Measures<br />
measures a fire-pro<strong>of</strong> (2 hour rating) (1 + 2) (1 + 2 + 5) (1A + 2 + 4) (3)<br />
K1 screen against adjoining or 3 or 3 or 3<br />
(1 hour rating) building or 4 or (4 + 5)<br />
K2 50 No special measures Interposition <strong>of</strong> a No special Measures 1A Measures 1<br />
K3 fire-pro<strong>of</strong> screen measures or 3 or 3<br />
(1 hour rating) or 4 or 4<br />
L3 No special measures<br />
Measure 1: Arrangements such that if the dielectric escapes from the transformer, it will be completely contained (in a sump, by sills around the<br />
transformer, and by blocking <strong>of</strong> cable trenches, ducts and so on, during construction).<br />
Measure 1A: In addition to measure 1, arrange that, in the event <strong>of</strong> liquid ignition there is no possibility <strong>of</strong> the fire spreading (any combustible<br />
material must be moved to a distance <strong>of</strong> at least 4 metres from the transformer, or at least 2 metres from it if a fire-pro<strong>of</strong> screen [<strong>of</strong> 1 hour rating] is<br />
interposed).<br />
Measure 2: Arrange that burning liquid will extinguish rapidly and naturally (by providing a pebble bed in the containment sump).<br />
Measure 3: An automatic device (gas, pressure & thermal relay, or Buchholz) for cutting <strong>of</strong>f the primary power supply, and giving an alarm, if gas<br />
appears in the transformer tank.<br />
Measure 4: Automatic fire-detection devices in close proximity to the transformer, for cutting <strong>of</strong>f primary power supply, and giving an alarm.<br />
Measure 5: Automatic closure by fire-pro<strong>of</strong> panels (1/2 hour minimum rating) <strong>of</strong> all openings (ventilation louvres, etc.) in the walls and ceiling <strong>of</strong><br />
the substation chamber.<br />
Notes:<br />
(1) A fire-pro<strong>of</strong> door (rated at 2 hours) is not considered to be an opening.<br />
(2) Transformer chamber adjoining a workshop and separated from it by walls, the fire-pro<strong>of</strong> characteristics <strong>of</strong> which are not rated for 2 hours.<br />
Areas situated in the middle <strong>of</strong> workshops the material being placed (or not) in a protective container.<br />
(3) It is indispensable that the equipment be enclosed in a chamber, the walls <strong>of</strong> which are solid, the only orifices being those necessary for<br />
ventilation purposes.<br />
Fig. B27 : Safety measures recommended in <strong>electrical</strong> <strong>installation</strong>s using dielectric liquids <strong>of</strong> classes 01, K1, K2 or K3<br />
The determination <strong>of</strong> optimal power<br />
Oversizing a transformer<br />
It results in:<br />
b Excessive investment and unecessarily high no-load losses, but<br />
b Lower on-load losses<br />
Undersizing a transformer<br />
It causes:<br />
b A reduced efficiency when fully loaded, (the highest efficiency is attained in the<br />
range 50% - 70% full load) so that the optimum loading is not achieved<br />
b On long-term overload, serious consequences for<br />
v The transformer, owing to the premature ageing <strong>of</strong> the windings insulation, and in<br />
extreme cases, resulting in failure <strong>of</strong> insulation and loss <strong>of</strong> the transformer<br />
v The <strong>installation</strong>, if overheating <strong>of</strong> the transformer causes protective relays to trip<br />
the controlling circuit-breaker.<br />
Definition <strong>of</strong> optimal power<br />
In order to select an optimal power (kVA) rating for a transformer, the following<br />
factors must be taken into account:<br />
b List the power <strong>of</strong> installed power-consuming equipment as described in <strong>Chapter</strong> A<br />
b Decide the utilization (or demand) factor for each individual item <strong>of</strong> load<br />
b Determine the load cycle <strong>of</strong> the <strong>installation</strong>, noting the duration <strong>of</strong> loads and overloads<br />
b Arrange for power-factor correction, if justified, in order to:<br />
v Reduce cost penalties in tariffs based, in part, on maximum kVA demand<br />
v Reduce the value <strong>of</strong> declared load (P(kVA) = P (kW)/cos ϕ)<br />
b Select, among the range <strong>of</strong> standard transformer ratings available, taking into<br />
account all possible future extensions to the <strong>installation</strong>.<br />
It is important to ensure that cooling arrangements for the transformer are adequate.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Fig. B28 : SM6 metal enclosed indoor MV eqpuipment<br />
4 The consumer substation<br />
with LV metering<br />
4.5 Instructions for use <strong>of</strong> MV equipment<br />
The purpose <strong>of</strong> this chapter is to provide general guidelines on how to avoid<br />
or greatly reduce MV equipment degradation on sites exposed to humidity and<br />
pollution.<br />
Normal service conditions for indoor MV equipment<br />
All MV equipments comply with specific standards and with the IEC 62271-1<br />
standard “Common specifications for high-voltage switchgear and controlgear”,<br />
which defines the normal conditions for the <strong>installation</strong> and use <strong>of</strong> such equipment.<br />
For instance, regarding humidity, the standard mentions:<br />
The conditions <strong>of</strong> humidity are as follows:<br />
b The average value <strong>of</strong> the relative humidity, measured over a period <strong>of</strong> 24 h does<br />
not exceed 90%;<br />
b The average value <strong>of</strong> the water vapour pressure, over a period <strong>of</strong> 24 h does not<br />
exceed 2.2 kPa;<br />
b The average value <strong>of</strong> the relative humidity, over a period <strong>of</strong> one month does not<br />
exceed 90%;<br />
b The average value <strong>of</strong> water vapour pressure, over a period <strong>of</strong> one month does not<br />
exceed 1.8 kPa;<br />
Under these conditions, condensation may occasionally occur.<br />
NOTE 1: Condensation can be expected where sudden temperature changes occur<br />
in period <strong>of</strong> high humidity.<br />
NOTE 2: To withstand the effects <strong>of</strong> high humidity and condensation, such as a<br />
breakdown <strong>of</strong> insulation or corrosion <strong>of</strong> metallic parts, switchgear <strong>design</strong>ed for such<br />
conditions and tested accordingly shoul be used.<br />
NOTE 3: Condensation may be prevented by special <strong>design</strong> <strong>of</strong> the building or<br />
housing, by suitable ventilation and heating <strong>of</strong> the station or by use <strong>of</strong> dehumifying<br />
equipment.<br />
As indicated in the standard, condensation may occasionally occur even under<br />
normal conditions. The standard goes on to indicate special measures concerning<br />
the substation premises that can be implemented to prevent condensation.<br />
Use under severe conditions<br />
Under certain severe conditions concerning humidity and pollution, largely beyond<br />
the normal conditions <strong>of</strong> use mentioned above, correctly <strong>design</strong>ed <strong>electrical</strong><br />
equipment can be subject to damage by rapid corrosion <strong>of</strong> metal parts and surface<br />
degradation <strong>of</strong> insulating parts.<br />
Remedial measures for condensation problems<br />
b Carefully <strong>design</strong> or adapt substation ventilation.<br />
b Avoid temperature variations.<br />
b Eliminate sources <strong>of</strong> humidity in the substation environment.<br />
b Install an air conditioning system.<br />
b Make sure cabling is in accordance with applicable <strong>rules</strong>.<br />
Remedial measures for pollution problems<br />
b Equip substation ventilation openings with chevron-type baffles to reduce entry<br />
<strong>of</strong> dust and pollution.<br />
b Keep substation ventilation to the minimum required for evacuation <strong>of</strong> transformer<br />
heat to reduce entry <strong>of</strong> pollution and dust.<br />
b Use MV cubicles with a sufficiently high degree <strong>of</strong> protection (IP).<br />
b Use air conditioning systems with filters to restrict entry <strong>of</strong> pollution and dust.<br />
b Regularly clean all traces <strong>of</strong> pollution from metal and insulating parts.<br />
Ventilation<br />
Substation ventilation is generally required to dissipate the heat produced by<br />
transformers and to allow drying after particularly wet or humid periods.<br />
However, a number <strong>of</strong> studies have shown that excessive ventilation can drastically<br />
increase condensation.<br />
Ventilation should therefore be kept to the minimum level required.<br />
Furthermore, ventilation should never generate sudden temperature variations that<br />
can cause the dew point to be reached.<br />
For this reason:<br />
Natural ventilation should be used whenever possible. If forced ventilation is<br />
necessary, the fans should operate continuously to avoid temperature fluctuations.<br />
Guidelines for sizing the air entry and exit openings <strong>of</strong> substations are presented<br />
hereafter.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B29<br />
© Schneider Electric - all rights reserved
B30<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
Fig. B29 : Natural ventilation<br />
200 mm<br />
mini<br />
Fig. B30 : Ventilation opening locations<br />
S<br />
S'<br />
H<br />
4 The consumer substation<br />
with LV metering<br />
Calculation methods<br />
A number <strong>of</strong> calculation methods are available to estimate the required size <strong>of</strong><br />
substation ventilation openings, either for the <strong>design</strong> <strong>of</strong> new substations or the<br />
adaptation <strong>of</strong> existing substations for which condensation problems have occurred.<br />
The basic method is based on transformer dissipation.<br />
The required ventilation opening surface areas S and S’ can be estimated using the<br />
following formulas:<br />
S � 1.8 x 10-4 P<br />
H<br />
and S'� 1.10 x S<br />
where:<br />
S = Lower (air entry) ventilation opening area [m²] (grid surface deducted)<br />
S’= Upper (air exit) ventilation opening area [m²] (grid surface deducted)<br />
P = Total dissipated power [W]<br />
P is the sum <strong>of</strong> the power dissipated by:<br />
b The transformer (dissipation at no load and due to load)<br />
b The LV switchgear<br />
b The MV switchgear<br />
H = Height between ventilation opening mid-points [m]<br />
See Fig. B29<br />
Note:<br />
This formula is valid for a yearly average temperature <strong>of</strong> 20 °C and a maximum<br />
altitude <strong>of</strong> 1,000 m.<br />
It must be noted that these formulae are able to determine only one order <strong>of</strong><br />
magnitude <strong>of</strong> the sections S and S', which are qualified as thermal section, i.e. fully<br />
open and just necessary to evacuate the thermal energy generated inside the MV/LV<br />
substation.<br />
The pratical sections are <strong>of</strong> course larger according ot the adopted technological<br />
solution.<br />
Indeed, the real air flow is strongly dependant:<br />
b on the openings shape and solutions adopted to ensure the cubicle protection<br />
index (IP): metal grid, stamped holes, chevron louvers,...<br />
b on internal components size and their position compared to the openings:<br />
transformer and/or retention oil box position and dimensions, flow channel between<br />
the components, ...<br />
b and on some physical and environmental parameters: outside ambient<br />
temperature, altitude, magnitude <strong>of</strong> the resulting temperature rise.<br />
The understanding and the optimization <strong>of</strong> the attached physical phenomena are<br />
subject to precise flow studies, based on the fluid dynamics laws, and realized with<br />
specific analytic s<strong>of</strong>tware.<br />
Example:<br />
Transformer dissipation = 7,970 W<br />
LV switchgear dissipation = 750 W<br />
MV switchgear dissipation = 300 W<br />
The height between ventilation opening mid-points is 1.5 m.<br />
Calculation:<br />
Dissipated Power P = 7,970 + 750 + 300 = 9,020 W<br />
S � 1.8 x 10-4 P<br />
1.5<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
� 1.32 m 2 and S'� 1.1 x 1.32 � 1.46 m 2<br />
Ventilation opening locations<br />
To favour evacuation <strong>of</strong> the heat produced by the transformer via natural convection,<br />
ventilation openings should be located at the top and bottom <strong>of</strong> the wall near the<br />
transformer. The heat dissipated by the MV switchboard is negligible.<br />
To avoid condensation problems, the substation ventilation openings should be<br />
located as far as possible from the switchboard (see Fig. B 30).
B - Connection to the MV public<br />
distribution network<br />
Fig. B31 : Chevron-blade baffles<br />
4 The consumer substation<br />
with LV metering<br />
Type <strong>of</strong> ventilation openings<br />
To reduce the entry <strong>of</strong> dust, pollution, mist, etc., the substation ventilation openings<br />
should be equipped with chevron-blade baffles.<br />
Always make sure the baffles are oriented in the right direction (see Fig. B31).<br />
Temperature variations inside cubicles<br />
To reduce temperature variations, always install anti-condensation heaters inside<br />
MV cubicles if the average relative humidity can remain high over a long period <strong>of</strong><br />
time. The heaters must operate continuously, 24 hours a day all year long.<br />
Never connect them to a temperature control or regulation system as this could lead<br />
to temperature variations and condensation as well as a shorter service life for the<br />
heating elements. Make sure the heaters <strong>of</strong>fer an adequate service life (standard<br />
versions are generally sufficient).<br />
Temperature variations inside the substation<br />
The following measures can be taken to reduce temperature variations inside the<br />
substation:<br />
b Improve the thermal insulation <strong>of</strong> the substation to reduce the effects <strong>of</strong> outdoor<br />
temperature variations on the temperature inside the substation.<br />
b Avoid substation heating if possible. If heating is required, make sure the regulation<br />
system and/or thermostat are sufficiently accurate and <strong>design</strong>ed to avoid excessive<br />
temperature swings (e.g. no greater than 1 °C).<br />
If a sufficiently accurate temperature regulation system is not available, leave the<br />
heating on continuously, 24 hours a day all year long.<br />
b Eliminate cold air drafts from cable trenches under cubicles or from openings in the<br />
substation (under doors, ro<strong>of</strong> joints, etc.).<br />
Substation environment and humidity<br />
Various factors outside the substation can affect the humidity inside.<br />
b Plants<br />
Avoid excessive plant growth around the substation.<br />
b Substation waterpro<strong>of</strong>ing<br />
The substation ro<strong>of</strong> must not leak. Avoid flat ro<strong>of</strong>s for which<br />
waterpro<strong>of</strong>ing is difficult to implement and maintain.<br />
b Humidity from cable trenches<br />
Make sure cable trenches are dry under all conditions.<br />
A partial solution is to add sand to the bottom <strong>of</strong> the cable trench.<br />
Pollution protection and cleaning<br />
Excessive pollution favours leakage current, tracking and flashover on insulators.<br />
To prevent MV equipment degradation by pollution, it is possible to either protect the<br />
equipment against pollution or regularly clean the resulting contamination.<br />
Protection<br />
Indoor MV switchgear can be protected by enclosures providing a sufficiently high<br />
degree <strong>of</strong> protection (IP).<br />
Cleaning<br />
If not fully protected, MV equipment must be cleaned regularly to prevent<br />
degradation by contamination from pollution.<br />
Cleaning is a critical process. The use <strong>of</strong> unsuitable products can irreversibly<br />
damage the equipment.<br />
For cleaning procedures, please contact your Schneider Electric correspondent.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B31<br />
© Schneider Electric - all rights reserved
B32<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
A consumer substation with MV metering<br />
is an <strong>electrical</strong> <strong>installation</strong> connected to a<br />
utility supply system at a nominal voltage <strong>of</strong><br />
1 kV - 35 kV and generally includes a single<br />
MV/LV transformer which exceeds 1,250 kVA,<br />
or several smaller transformers.<br />
The rated current <strong>of</strong> the MV switchgear does<br />
not normally exceed 400 A.<br />
5 The consumer substation<br />
with MV metering<br />
5.1 <strong>General</strong><br />
Functions<br />
The substation<br />
According to the complexity <strong>of</strong> the <strong>installation</strong> and the manner in which the load is<br />
divided, the substation:<br />
b Might include one room containing the MV switchboard and metering panel(s),<br />
together with the transformer(s) and low-voltage main distribution board(s),<br />
b Or might supply one or more transformer rooms, which include local LV distribution<br />
boards, supplied at MV from switchgear in a main substation, similar to that<br />
described above.<br />
These substations may be installed, either:<br />
b Inside a building, or<br />
b Outdoors in prefabricated housings.<br />
Connection to the MV network<br />
Connection at MV can be:<br />
b Either by a single service cable or overhead line, or<br />
b Via two mechanically interlocked load-break switches with two service cables from<br />
duplicate supply feeders, or<br />
b Via two load-break switches <strong>of</strong> a ring-main unit.<br />
Metering<br />
Before the <strong>installation</strong> project begins, the agreement <strong>of</strong> the power-supply utility<br />
regarding metering arrangements must be obtained.<br />
A metering panel will be incorporated in the MV switchboard. Voltage transformers<br />
and current transformers, having the necessary metering accuracy, may be included<br />
in the main incoming circuit-breaker panel or (in the case <strong>of</strong> the voltage transformer)<br />
may be installed separately in the metering panel.<br />
Transformer rooms<br />
If the <strong>installation</strong> includes a number <strong>of</strong> transformer rooms, MV supplies from the main<br />
substation may be by simple radial feeders connected directly to the transformers, or<br />
by duplicate feeders to each room, or again, by a ring-main, according to the degree<br />
<strong>of</strong> supply availability desired.<br />
In the two latter cases, 3-panel ring-main units will be required at each transformer<br />
room.<br />
Local emergency generators<br />
Emergency standby generators are intended to maintain a power supply to essential<br />
loads, in the event <strong>of</strong> failure <strong>of</strong> the power supply system.<br />
Capacitors<br />
Capacitors will be installed, according to requirements:<br />
b In stepped MV banks at the main substation, or<br />
b At LV in transformer rooms.<br />
Transformers<br />
For additional supply-security reasons, transformers may be arranged for automatic<br />
changeover operation, or for parallel operation.<br />
One-line diagrams<br />
The diagrams shown in Figure B32 next page represent:<br />
b The different methods <strong>of</strong> MV service connection, which may be one <strong>of</strong> four types:<br />
v Single-line service<br />
v Single-line service (equipped for extension to form a ring main)<br />
v Duplicate supply service<br />
v Ring main service<br />
b <strong>General</strong> protection at MV, and MV metering functions<br />
b Protection <strong>of</strong> outgoing MV circuits<br />
b Protection <strong>of</strong> LV distribution circuits<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
Power supply<br />
system<br />
Service connection MV protection<br />
and metering<br />
Supplier/consumer<br />
interface<br />
Single-line service<br />
Single-line service<br />
(equipped for<br />
extension to form<br />
a ring main)<br />
Duplicatesupply<br />
service<br />
Ring-main<br />
service<br />
Fig. B32 : Consumer substation with MV metering<br />
5 The consumer substation<br />
with MV metering<br />
MV distribution and protection<br />
<strong>of</strong> outgoing circuits<br />
Downstream terminals <strong>of</strong><br />
MV isolator for the <strong>installation</strong><br />
A single transformer<br />
Protection<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
LV distribution<br />
and protection<br />
LV terminals <strong>of</strong><br />
transformer<br />
Protection<br />
LV<br />
Automatic LV/MV<br />
standby source<br />
Protection<br />
+ automatic<br />
changeover<br />
feature<br />
Automatic LV<br />
standby source<br />
B33<br />
© Schneider Electric - all rights reserved
B34<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
MV distribution<br />
panels for<br />
which standby<br />
supply is<br />
required<br />
Automatic<br />
changeover<br />
panel<br />
Busbar<br />
transition<br />
panel<br />
From standby generator<br />
P y 20,000 kVA<br />
To remainder<br />
<strong>of</strong> the MV<br />
switchboard<br />
Fig. B33 : Section <strong>of</strong> MV switchboard including standby supply<br />
panel<br />
5 The consumer substation<br />
with MV metering<br />
5.2 Choice <strong>of</strong> panels<br />
A substation with MV metering includes, in addition to the panels described in 4.2,<br />
panels specifically <strong>design</strong>ed for metering and, if required, for automatic or manual<br />
changeover from one source to another.<br />
Metering and general protection<br />
These two functions are achieved by the association <strong>of</strong> two panels:<br />
b One panel containing the VT<br />
b The main MV circuit-breaker panel containing the CTs for measurement and<br />
protection<br />
The general protection is usually against overcurrent (overload and short-circuit) and<br />
earth faults. Both schemes use protective relays which are sealed by the powersupply<br />
utility.<br />
Substation including generators<br />
Generator in stand alone operation<br />
If the <strong>installation</strong> needs great power supply availability, a MV standby generator set<br />
can be used. In such a case, the <strong>installation</strong> must include an automatic changeover.<br />
In order to avoid any posssibility <strong>of</strong> parallel operation <strong>of</strong> the generator with the power<br />
supply network, a specific panel with automatic changeover is needed (see Fig. B33).<br />
b Protection<br />
Specific protective devices are intended to protect the generator itself. It must be<br />
noted that, due to the very low short-circuit power <strong>of</strong> the generator comparing with<br />
the power supply network, a great attention must be paid to protection discrimination.<br />
b Control<br />
A voltage regulator controlling an alternator is generally arranged to respond to a<br />
reduction <strong>of</strong> voltage at its terminals by automatically increasing the excitation current<br />
<strong>of</strong> the alternator, until the voltage is restored to normal. When it is intended that<br />
the alternator should operate in parallel with others, the AVR (Automatic Voltage<br />
Regulator) is switched to “parallel operation” in which the AVR control circuit is<br />
slightly modified (compounded) to ensure satisfactory sharing <strong>of</strong> kvars with the other<br />
parallel machines.<br />
When a number <strong>of</strong> alternators are operating in parallel under AVR control, an<br />
increase in the excitation current <strong>of</strong> one <strong>of</strong> them (for example, carried out manually<br />
after switching its AVR to Manual control) will have practically no effect on the voltage<br />
level. In fact, the alternator in question will simply operate at a lower power factor<br />
(more kVA, and therefore more current) than before.<br />
The power factor <strong>of</strong> all the other machines will automatically improve, such that the<br />
load power factor requirements are satisfied, as before.<br />
Generator operating in parallel with the utility supply network<br />
To connect a generator set on the network, the agreement <strong>of</strong> the power supply utility<br />
is usually required. <strong>General</strong>ly the equipement (panels, protection relays) must be<br />
approved by the utility.<br />
The following notes indicate some basic consideration to be taken into account for<br />
protection and control.<br />
b Protection<br />
To study the connection <strong>of</strong> generator set, the power supply utility needs some data<br />
as follows :<br />
v Power injected on the network<br />
v Connection mode<br />
v Short-circuit current <strong>of</strong> the generator set<br />
v Voltage unbalance <strong>of</strong> the generator<br />
v etc.<br />
Depending on the connection mode, dedicated uncoupling protection functions are<br />
required :<br />
v Under-voltage and over-voltage protection<br />
v Under-frequency and over-frequency protection<br />
v Zero sequence overvoltage protection<br />
v Maximum time <strong>of</strong> coupling (for momentary coupling)<br />
v Reverse real power<br />
For safety reasons, the switchgear used for uncoupling must also be provided<br />
with the characteristics <strong>of</strong> a disconnector (i.e total isolation <strong>of</strong> all active conductors<br />
between the generator set and the power supply network).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
B - Connection to the MV public<br />
distribution network<br />
5 The consumer substation<br />
with MV metering<br />
b Control<br />
When generators at a consumer’s substation operate in parallel with all the<br />
generation <strong>of</strong> the utility power supply system, supposing the power system voltage is<br />
reduced for operational reasons (it is common to operate MV systems within a range<br />
<strong>of</strong> ± 5% <strong>of</strong> nominal voltage, or even more, where load-flow patterns require it), an<br />
AVR set to maintain the voltage within ± 3% (for example) will immediately attempt to<br />
raise the voltage by increasing the excitation current <strong>of</strong> the alternator.<br />
Instead <strong>of</strong> raising the voltage, the alternator will simply operate at a lower power<br />
factor than before, thereby increasing its current output, and will continue to do so,<br />
until it is eventually tripped out by its overcurrent protective relays. This is a wellknown<br />
problem and is usually overcome by the provision <strong>of</strong> a “constant power-<br />
factor” control switch on the AVR unit.<br />
By making this selection, the AVR will automatically adjust the excitation current<br />
to match whatever voltage exists on the power system, while at the same time<br />
maintaining the power factor <strong>of</strong> the alternator constant at the pre-set value (selected<br />
on the AVR control unit).<br />
In the event that the alternator becomes decoupled from the power system, the AVR<br />
must be automatically (rapidly) switched back to “constant-voltage” control.<br />
5.3 Parallel operation <strong>of</strong> transformers<br />
The need for operation <strong>of</strong> two or more transformers in parallel <strong>of</strong>ten arises due to:<br />
b Load growth, which exceeds the capactiy <strong>of</strong> an existing transformer<br />
b Lack <strong>of</strong> space (height) for one large transformer<br />
b A measure <strong>of</strong> security (the probability <strong>of</strong> two transformers failing at the same time<br />
is very small)<br />
b The adoption <strong>of</strong> a standard size <strong>of</strong> transformer throughout an <strong>installation</strong><br />
Total power (kVA)<br />
The total power (kVA) available when two or more transformers <strong>of</strong> the same<br />
kVA rating are connected in parallel, is equal to the sum <strong>of</strong> the individual ratings,<br />
providing that the percentage impedances are all equal and the voltage ratios are<br />
identical.<br />
Transformers <strong>of</strong> unequal kVA ratings will share a load practically (but not exactly)<br />
in proportion to their ratings, providing that the voltage ratios are identical and the<br />
percentage impedances (at their own kVA rating) are identical, or very nearly so.<br />
In these cases, a total <strong>of</strong> more than 90% <strong>of</strong> the sum <strong>of</strong> the two ratings is normally<br />
available.<br />
It is recommended that transformers, the kVA ratings <strong>of</strong> which differ by more<br />
than 2:1, should not be operated permanently in parallel.<br />
Conditions necessary for parallel operation<br />
All paralleled units must be supplied from the same network.<br />
The inevitable circulating currents exchanged between the secondary circuits <strong>of</strong><br />
paralleled transformers will be negligibly small providing that:<br />
b Secondary cabling from the transformers to the point <strong>of</strong> paralleling have<br />
approximately equal lengths and characteristics<br />
b The transformer manufacturer is fully informed <strong>of</strong> the duty intended for the<br />
transformers, so that:<br />
v The winding configurations (star, delta, zigzag star) <strong>of</strong> the several transformers<br />
have the same phase change between primary and secondary voltages<br />
v The short-circuit impedances are equal, or differ by less than 10%<br />
v Voltage differences between corresponding phases must not exceed 0.4%<br />
v All possible information on the conditions <strong>of</strong> use, expected load cycles, etc. should<br />
be given to the manufacturer with a view to optimizing load and no-load losses<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B35<br />
© Schneider Electric - all rights reserved
B36<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
5 The consumer substation<br />
with MV metering<br />
Common winding arrangements<br />
As described in 4.4 “Electrical characteristics-winding configurations” the<br />
relationships between primary, secondary, and tertiary windings depend on:<br />
b Type <strong>of</strong> windings (delta, star, zigzag)<br />
b Connection <strong>of</strong> the phase windings<br />
Depending on which ends <strong>of</strong> the windings form the star point (for example), a<br />
star winding will produce voltages which are 180° displaced with respect to those<br />
produced if the opposite ends had been joined to form the star point. Similar 180°<br />
changes occur in the two possible ways <strong>of</strong> connecting phase-to-phase coils to form<br />
delta windings, while four different combinations <strong>of</strong> zigzag connections are possible.<br />
b The phase displacement <strong>of</strong> the secondary phase voltages with respect to the<br />
corresponding primary phase voltages.<br />
As previously noted, this displacement (if not zero) will always be a multiple <strong>of</strong><br />
30° and will depend on the two factors mentioned above, viz type <strong>of</strong> windings and<br />
connection (i.e. polarity) <strong>of</strong> the phase windings.<br />
By far the most common type <strong>of</strong> distribution transformer winding configuration is the<br />
Dyn 11 connection (see Fig. B34).<br />
Voltage vectors<br />
1<br />
Fig. B34 : Phase change through a Dyn 11 transformer<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
3<br />
1<br />
2<br />
3<br />
V 12<br />
Windings<br />
correspondence<br />
V 12 on the primary winding produces V 1N in the<br />
secondary winding and so on ...<br />
2<br />
1<br />
3<br />
N<br />
N<br />
2<br />
1<br />
2<br />
3
B - Connection to the MV public<br />
distribution network<br />
6 Constitution <strong>of</strong><br />
MV/LV distribution substations<br />
MV/LV substations are constructed according to the magnitude <strong>of</strong> the load and the<br />
kind <strong>of</strong> power system in question.<br />
Substations may be built in public places, such as parks, residential districts, etc. or<br />
on private premises, in which case the power supply authority must have unrestricted<br />
access. This is normally assured by locating the substation, such that one <strong>of</strong> its<br />
walls, which includes an access door, coincides with the boundary <strong>of</strong> the consumers<br />
premises and the public way.<br />
6.1 Different types <strong>of</strong> substation<br />
Substations may be classified according to metering arrangements (MV or LV) and<br />
type <strong>of</strong> supply (overhead line or underground cable).<br />
The substations may be installed:<br />
b Either indoors in room specially built for the purpose, within a building, or<br />
b An outdoor <strong>installation</strong> which could be :<br />
v Installed in a dedicated enclosure prefabricated or not, with indoor equipment<br />
(switchgear and transformer)<br />
v Ground mounted with outdoor equipment (switchgear and transformers)<br />
v Pole mounted with dedicated outdoor equipment (swithgear and transformers)<br />
Prefabricated substations provide a particularly simple, rapid and competitive choice.<br />
6.2 Indoor substation<br />
Conception<br />
Figure B35 shows a typical equipment layout recommended for a LV metering<br />
substation.<br />
Remark: the use <strong>of</strong> a cast-resin dry-type transformer does not need a fireprotection<br />
oil sump. However, periodic cleaning is needed.<br />
MV connections to transformer<br />
(included in a panel or free-standing)<br />
2 incoming<br />
MV panels<br />
MV<br />
switching<br />
and<br />
protection<br />
panel<br />
Connection to the powersupply<br />
network by single-core<br />
or three-core cables,<br />
with or without a cable trench<br />
Fig. B35 : Typical arrangment <strong>of</strong> switchgear panels for LV metering<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
LV connections<br />
from<br />
transformer<br />
LV switchgear<br />
Current<br />
transformers<br />
provided by<br />
power-supply<br />
authority<br />
Transformer Oil sump LV cable<br />
trench<br />
B37<br />
© Schneider Electric - all rights reserved
B38<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
6 Constitution <strong>of</strong><br />
MV/LV distribution substations<br />
Service connections and equipment interconnections<br />
At high voltage<br />
b Connections to the MV system are made by, and are the responsibility <strong>of</strong> the utility<br />
b Connections between the MV switchgear and the transformers may be:<br />
v By short copper bars where the transformer is housed in a panel forming part <strong>of</strong><br />
the MV switchboard<br />
v By single-core screened cables with synthetic insulation, with possible use <strong>of</strong> plugin<br />
type terminals at the transformer<br />
At low voltage<br />
b Connections between the LV terminals <strong>of</strong> the transformer and the LV switchgear<br />
may be:<br />
v Single-core cables<br />
v Solid copper bars (circular or rectangular section) with heat-shrinkable insulation<br />
Metering (see Fig. B36)<br />
b Metering current transformers are generally installed in the protective cover <strong>of</strong> the<br />
power transformer LV terminals, the cover being sealed by the supply utility<br />
b Alternatively, the current transformers are installed in a sealed compartment within<br />
the main LV distribution cabinet<br />
b The meters are mounted on a panel which is completely free from vibrations<br />
b Placed as close to the current transformers as possible, and<br />
b Are accessible only to the utility<br />
100<br />
MV supply LV distribution<br />
Fig. B36 : Plan view <strong>of</strong> typical substation with LV metering<br />
Earthing circuits<br />
Common earth busbar<br />
for the substation<br />
The substation must include:<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Safety accessories<br />
800 mini<br />
Meters<br />
b An earth electrode for all exposed conductive parts <strong>of</strong> <strong>electrical</strong> equipment in the<br />
substation and exposed extraneous metal including:<br />
v Protective metal screens<br />
v Reinforcing rods in the concrete base <strong>of</strong> the substation<br />
Substation lighting<br />
Supply to the lighting circuits can be taken from a point upstream or downstream<br />
<strong>of</strong> the main incoming LV circuit-breaker. In either case, appropriate overcurrent<br />
protection must be provided. A separate automatic circuit (or circuits) is (are)<br />
recommended for emergency lighting purposes.<br />
Operating switches, pushbuttons, etc. are normally located immediately adjacent to<br />
entrances.<br />
Lighting fittings are arranged such that:<br />
b Switchgear operating handles and position indication markings are adequately<br />
illuminated<br />
b All metering dials and instruction plaques and so on, can be easily read
B - Connection to the MV public<br />
distribution network<br />
Walk-in Non walk-in Half buried<br />
a - b -<br />
Underground<br />
Fig. B38 : The four <strong>design</strong>s according to IEC 62271-202<br />
standard and two pictures [a] walk-in type MV/LV substation;<br />
[b] half buried type MV/LV substation<br />
6 Constitution <strong>of</strong><br />
MV/LV distribution substations<br />
Materials for operation and safety<br />
According to local safety <strong>rules</strong>, generally, the substation is provided with:<br />
b Materials for assuring safe exploitation <strong>of</strong> the equipment including:<br />
v Insulating stool and/or an insulating mat (rubber or synthetic)<br />
v A pair <strong>of</strong> insulated gloves stored in an envelope provided for the purpose<br />
v A voltage-detecting device for use on the MV equipment<br />
v Earthing attachments (according to type <strong>of</strong> switchgear)<br />
b Fire-extinguishing devices <strong>of</strong> the powder or CO2 type<br />
b Warning signs, notices and safety alarms:<br />
v On the external face <strong>of</strong> all access doors, a DANGER warning plaque and<br />
prohibition <strong>of</strong> entry notice, together with instructions for first-aid care for victims <strong>of</strong><br />
<strong>electrical</strong> accidents.<br />
6.3 Outdoor substations<br />
Outdoor substation with prefabricated enclosures<br />
A prefabricated MV/LV substation complying with IEC 62271-202 standard includes :<br />
b equipement in accordance with IEC standards<br />
b a type tested enclosure, which means during its <strong>design</strong>, it has undergone a battery<br />
<strong>of</strong> tests (see Fig. B37):<br />
v Degree <strong>of</strong> protection<br />
v Functional tests<br />
v Temperature class<br />
v Non-flammable materials<br />
v Mechanical resistance <strong>of</strong> the enclosure<br />
v Sound level<br />
v Insulation level<br />
v Internal arc withstand<br />
v Earthing circuit test<br />
v Oil retention,…<br />
Use <strong>of</strong> equipment conform<br />
to IEC standards:<br />
b Degree <strong>of</strong> protection<br />
b Electromagnetic<br />
compatibility<br />
b Functional tests<br />
b Temperature class<br />
b Non-flammable<br />
materials<br />
Fig. B37 : Type tested substation according to IEC 62271-202 standard<br />
Main benefits are :<br />
b Safety:<br />
v For public and operators thanks to a high reproducible quality level<br />
b Cost effective:<br />
v Manufactured, equipped and tested in the factory<br />
b Delivery time<br />
v Delivered ready to be connected.<br />
IEC 62271-202 standard includes four main <strong>design</strong>s (see Fig. B38)<br />
b Walk-in type substation :<br />
v Operation protected from bad weather conditions<br />
b Non walk-in substation<br />
v Ground space savings, and outdoors operations<br />
b Half buried substation<br />
v Limited visual impact<br />
b Underground substation<br />
v Blends completely into the environment.<br />
LV<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
MV<br />
Earthing circuit test Oil retention<br />
Mechanical resistance<br />
<strong>of</strong> the enclosure:<br />
b Sound level<br />
b Insulation level<br />
b Internal arcing<br />
withstand<br />
B39<br />
© Schneider Electric - all rights reserved
B40<br />
© Schneider Electric - all rights reserved<br />
B - Connection to the MV public<br />
distribution network<br />
Fig. B39 : Outdoor substations without enclosures<br />
6 Constitution <strong>of</strong><br />
MV/LV distribution substations<br />
Outdoor substations without enclosures (see Fig. B39)<br />
These kinds <strong>of</strong> outdoor substation are common in some countries, based on<br />
weatherpro<strong>of</strong> equipment exposed to the elements.<br />
These substations comprise a fenced area in which three or more concrete plinths<br />
are installed for:<br />
b A ring-main unit, or one or more switch-fuse or circuit-breaker unit(s)<br />
b One or more transformer(s), and<br />
b One or more LV distribution panel(s).<br />
Pole mounted substations<br />
Field <strong>of</strong> application<br />
These substations are mainly used to supply isolated rural consumers from MV<br />
overhead line distribution systems.<br />
Constitution<br />
In this type <strong>of</strong> substation, most <strong>of</strong>ten, the MV transformer protection is provided by<br />
fuses.<br />
Lightning arresters are provided, however, to protect the transformer and consumers<br />
as shown in Figure B40.<br />
<strong>General</strong> arrangement <strong>of</strong> equipment<br />
As previously noted the location <strong>of</strong> the substation must allow easy access, not only<br />
for personnel but for equipment handling (raising the transformer, for example) and<br />
the manœuvring <strong>of</strong> heavy vehicles.<br />
LV circuit breaker D1<br />
Safety earth mat<br />
Fig. B40 : Pole-mounted transformer substation<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Lightning<br />
arresters<br />
Earthing conductor 25 mm 2 copper<br />
Protective conductor cover
2<br />
<strong>Chapter</strong> C<br />
Connection to the LV utility<br />
distribution network<br />
Contents<br />
Low-voltage utility distribution networks C2<br />
1.1 Low-voltage consumers C2<br />
1.2 Low-voltage distribution networks C10<br />
1.3 The consumer service connection C11<br />
1.4 Quality <strong>of</strong> supply voltage C15<br />
Tariffs and metering C 6<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
C<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
C2<br />
C - Connecion to the LV public<br />
distribution network<br />
The most-common LV supplies are within the<br />
range 120 V single phase to 240/415 V<br />
3-phase 4-wires.<br />
Loads up to 250 kVA can be supplied at LV, but<br />
power-supply organizations generally propose<br />
a MV service at load levels for which their<br />
LV networks are marginally adequate.<br />
An international voltage standard for 3-phase<br />
4-wire LV systems is recommended by the<br />
IEC 60038 to be 230/400 V<br />
Low-voltage utility distribution<br />
networks<br />
. Low-voltage consumers<br />
In Europe, the transition period on the voltage tolerance to “230V/400V + 10% / - 10%”<br />
has been extended for another 5 years up to the year 2008.<br />
Low-voltage consumers are, by definition, those consumers whose loads can be<br />
satisfactorily supplied from the low-voltage system in their locality.<br />
The voltage <strong>of</strong> the local LV network may be 120/208 V or 240/415 V, i.e. the lower<br />
or upper extremes <strong>of</strong> the most common 3-phase levels in general use, or at some<br />
intermediate level, as shown in Figure C .<br />
An international voltage standard for 3-phase 4-wire LV systems is recommended<br />
by the IEC 60038 to be 230/400 V.<br />
Loads up to 250 kVA can be supplied at LV, but power-supply organizations<br />
generally propose a MV service at load levels for which their LV networks are<br />
marginally adequate.<br />
Country Frequency & tolerance Domestic (V) Commercial (V) Industrial (V)<br />
(Hz & %)<br />
Afghanistan 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k)<br />
Algeria 50 ± 1.5 220/127 (e) 380/220 (a) 10,000<br />
220 (k) 220/127 (a) 5,500<br />
6,600<br />
380/220 (a)<br />
Angola 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k)<br />
Antigua and Barbuda 60 240 (k) 400/230 (a) 400/230 (a)<br />
120 (k) 120/208 (a) 120/208 (a)<br />
Argentina 50 ± 2 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Armenia 50 ± 5 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Australia 50 ± 0.1 415/240 (a) 415/240 (a) 22,000<br />
240 (k) 440/250 (a) 11,000<br />
440 (m) 6,600<br />
415/240<br />
440/250<br />
Austria 50 ± 0.1 230 (k) 380/230 (a) (b) 5,000<br />
230 (k) 380/220 (a)<br />
Azerbaijan 50 ± 0.1 208/120 (a) 208/120 (a)<br />
240/120 (k) 240/120 (k)<br />
Bahrain 50 ± 0.1 415/240 (a) 415/240 (a) 11,000<br />
240 (k) 240 (k) 415/240 (a)<br />
240 (k)<br />
Bangladesh 50 ± 2 410/220 (a) 410/220 (a) 11,000<br />
220 (k) 410/220 (a)<br />
Barbados 50 ± 6 230/115 (j) 230/115 (j) 230/400 (g)<br />
115 (k) 200/115 (a) 230/155 (j)<br />
220/115 (a)<br />
Belarus 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
220/127 (a)<br />
127 (k)<br />
Belgium 50 ± 5 230 (k) 230 (k) 6,600<br />
230 (a) 230 (a) 10,000<br />
3N, 400 3N, 400 11,000<br />
15,000<br />
Bolivia 50 ± 0.5 230 (k) 400/230 (a) 400/230 (a)<br />
230 (k)<br />
Botswana 50 ± 3 220 (k) 380/220 (a) 380/220 (a)<br />
Brazil 60 220 (k) 220/380 (a) 13,800<br />
127 (k) 127/220 (a) 11,200<br />
220/380 (a)<br />
127/220 (a)<br />
Brunei 50 ± 2 230 230 11,000<br />
68,000<br />
Bulgaria 50 ± 0.1 220 220/240 1,000<br />
690<br />
380<br />
Fig. C1 : Voltage <strong>of</strong> local LV network and their associated circuit diagrams (continued on next page)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
C - Connecion to the LV public<br />
distribution network<br />
Fig. C1 : Voltage <strong>of</strong> local LV network and their associated circuit diagrams (continued on next page)<br />
Low-voltage utility distribution<br />
networks<br />
Country Frequency & tolerance<br />
(Hz & %)<br />
Domestic (V) Commercial (V) Industrial (V)<br />
Cambodia 50 ± 1 220 (k) 220/300 220/380<br />
Cameroon 50 ± 1 220/260 (k) 220/260 (k) 220/380 (a)<br />
Canada 60 ± 0.02 120/240 (j) 347/600 (a) 7,200/12,500<br />
480 (f) 347/600 (a)<br />
240 (f) 120/208<br />
120/240 (j) 600 (f)<br />
120/208 (a) 480 (f)<br />
240 (f)<br />
Cape Verde 220 220 380/400<br />
Chad 50 ± 1 220 (k) 220 (k) 380/220 (a)<br />
Chile 50 ± 1 220 (k) 380/220 (a) 380/220 (a)<br />
China 50 ± 0.5 220 (k) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Colombia 60 ± 1 120/240 (g) 120/240 (g) 13,200<br />
120 (k) 120 (k) 120/240 (g)<br />
Congo 50 220 (k) 240/120 (j)<br />
120 (k)<br />
380/220 (a)<br />
Croatia 50 400/230 (a) 400/230 (a) 400/230 (a)<br />
230 (k) 230 (k)<br />
Cyprus 50 ± 0.1 240 (k) 415/240 11,000<br />
415/240<br />
Czech Republic 50 ± 1 230 500 400,000<br />
230/400 220,000<br />
110,000<br />
35,000<br />
22,000<br />
10,000<br />
6,000<br />
3,000<br />
Denmark 50 ± 1 400/230 (a) 400/230 (a) 400/230 (a)<br />
Djibouti 50 400/230 (a) 400/230 (a)<br />
Dominica 50 230 (k) 400/230 (a) 400/230 (a)<br />
Egypt 50 ± 0.5 380/220 (a) 380/220 (a) 66,000<br />
220 (k) 220 (k) 33,000<br />
20,000<br />
11,000<br />
6,600<br />
380/220 (a)<br />
Estonia 50 ± 1 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Ethiopia 50 ± 2.5 220 (k) 380/231 (a) 15 000<br />
380/231 (a)<br />
Falkland Islands 50 ± 3 230 (k) 415/230 (a) 415/230 (a)<br />
Fidji Islands 50 ± 2 415/240 (a) 415/240 (a) 11,000<br />
240 (k) 240 (k) 415/240 (a)<br />
Finland 50 ± 0.1 230 (k) 400/230 (a) 690/400 (a)<br />
400/230 (a)<br />
France 50 ± 1 400/230 (a) 400/230 20,000<br />
230 (a) 690/400 10,000<br />
590/100 230/400<br />
Gambia 50 220 (k) 220/380 380<br />
Georgia 50 ± 0.5 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Germany 50 ± 0.3 400/230 (a) 400/230 (a) 20,000<br />
230 (k) 230 (k) 10,000<br />
6,000<br />
690/400<br />
400/230<br />
Ghana 50 ± 5 220/240 220/240 415/240 (a)<br />
Gibraltar 50 ± 1 415/240 (a) 415/240 (a) 415/240 (a)<br />
Greece 50 220 (k) 6,000 22,000<br />
230 380/220 (a) 20,000<br />
15,000<br />
6,600<br />
Granada 50 230 (k) 400/230 (a) 400/230 (a)<br />
Hong Kong 50 ± 2 220 (k) 380/220 (a) 11,000<br />
220 (k) 386/220 (a)<br />
Hungary 50 ± 5 220 220 220/380<br />
Iceland 50 ± 0.1 230 230/400 230/400<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
C<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
C<br />
C - Connecion to the LV public<br />
distribution network<br />
Fig. C1 : Voltage <strong>of</strong> local LV network and their associated circuit diagrams (continued on next page)<br />
Low-voltage utility distribution<br />
networks<br />
Country Frequency & tolerance Domestic (V) Commercial (V) Industrial (V)<br />
(Hz & %)<br />
India 50 ± 1.5 440/250 (a) 440/250 (a) 11,000<br />
230 (k) 230 (k) 400/230 (a)<br />
440/250 (a)<br />
Indonesia 50 ± 2 220 (k) 380/220 (a) 150,000<br />
20,000<br />
380/220 (a)<br />
Iran 50 ± 5 220 (k) 380/220 (a) 20,000<br />
11,000<br />
400/231 (a)<br />
380/220 (a)<br />
Iraq 50 220 (k) 380/220 (a) 11,000<br />
6,600<br />
3,000<br />
380/220 (a)<br />
Ireland 50 ± 2 230 (k) 400/230 (a) 20,000<br />
10,000<br />
400/230 (a)<br />
Israel 50 ± 0.2 400/230 (a) 400/230 (a) 22,000<br />
230 (k) 230 (k) 12,600<br />
6,300<br />
400/230 (a)<br />
Italy 50 ± 0.4 400/230 (a) 400/230 (a) 20,000<br />
230 (k) 15,000<br />
10,000<br />
400/230 (a)<br />
Jamaica 50 ± 1 220/110 (g) (j) 220/110 (g) (j) 4,000<br />
2,300<br />
220/110 (g)<br />
Japan (east) + 0.1 200/100 (h) 200/100 (h) 140,000<br />
- 0.3 (up to 50 kW) 60,000<br />
20,000<br />
6,000<br />
200/100 (h)<br />
Jordan 50 380/220 (a) 380/220 (a) 400 (a)<br />
400/230 (k)<br />
Kazakhstan 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
220/127 (a)<br />
127 (k)<br />
Kenya 50 240 (k) 415/240 (a) 415/240 (a)<br />
Kirghizia 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
220/127 (a)<br />
127 (k)<br />
Korea (North) 60 +0, -5 220 (k) 220/380 (a) 13,600<br />
6,800<br />
Korea (South) 60 100 (k) 100/200 (j)<br />
Kuwait 50 ± 3 240 (k) 415/240 (a) 415/240 (a)<br />
Laos 50 ± 8 380/220 (a) 380/220 (a) 380/220 (a)<br />
Lesotho 220 (k) 380/220 (a) 380/220 (a)<br />
Latvia 50 ± 0.4 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Lebanon 50 220 (k) 380/220 (a) 380/220 (a)<br />
Libya 50 230 (k) 400/230 (a) 400/230 (a)<br />
127 (k) 220/127 (a) 220/127 (a)<br />
230 (k)<br />
127 (k)<br />
Lithuania 50 ± 0.5 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Luxembourg 50 ± 0.5 380/220 (a) 380/220 (a) 20,000<br />
15,000<br />
5,000<br />
Macedonia 50 380/220 (a) 380/220 (a) 10,000<br />
220 (k) 220 (k) 6,600<br />
380/220 (a)<br />
Madagascar 50 220/110 (k) 380/220 (a) 35,000<br />
5,000<br />
380/220<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
C - Connecion to the LV public<br />
distribution network<br />
Fig. C1 : Voltage <strong>of</strong> local LV network and their associated circuit diagrams (continued on next page)<br />
Low-voltage utility distribution<br />
networks<br />
Country Frequency & tolerance<br />
(Hz & %)<br />
Domestic (V) Commercial (V) Industrial (V)<br />
Malaysia 50 ± 1 240 (k)<br />
415 (a)<br />
415/240 (a) 415/240 (a)<br />
Malawi 50 ± 2.5 230 (k) 400 (a)<br />
230 (k)<br />
400 (a)<br />
Mali 50 220 (k) 380/220 (a) 380/220 (a)<br />
127 (k) 220/127 (a)<br />
220 (k)<br />
127 (k)<br />
220/127 (a)<br />
Malta 50 ± 2 240 (k) 415/240 (a) 415/240 (a)<br />
Martinique 50 127 (k) 220/127 (a)<br />
127 (k)<br />
220/127 (a)<br />
Mauritania 50 ± 1 230 (k) 400/230 (a) 400/230 (a)<br />
Mexico 60 ± 0.2 127/220 (a) 127/220 (a) 13,800<br />
220 (k) 220 (k) 13,200<br />
120 (l) 120 (l) 277/480 (a)<br />
127/220 (b)<br />
Moldavia 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k)<br />
220/127 (a)<br />
127 (k)<br />
220 (k)<br />
Morocco 50 ± 5 380/220 (a) 380/220 (a) 225,000<br />
220/110 (a) 150,000<br />
60,000<br />
22,000<br />
20,000<br />
Mozambique 50 380/220 (a) 380/220 (a) 6,000<br />
10,000<br />
Nepal 50 ± 1 220 (k) 440/220 (a) 11,000<br />
220 (k) 440/220 (a)<br />
Netherlands 50 ± 0.4 230/400 (a) 230/400 (a) 25,000<br />
230 (k) 20,000<br />
12,000<br />
10,000<br />
230/400<br />
New Zealand 50 ± 1.5 400/230 (e) (a) 400/230 (e) (a) 11,000<br />
230 (k)<br />
460/230 (e)<br />
230 (k) 400/230 (a)<br />
Niger 50 ± 1 230 (k) 380/220 (a) 15,000<br />
380/220 (a)<br />
Nigeria 50 ± 1 230 (k) 400/230 (a) 15,000<br />
220 (k) 380/220 (a) 11,000<br />
400/230 (a)<br />
380/220 (a)<br />
Norway 50 ± 2 230/400 230/400 230/400<br />
690<br />
Oman 50 240 (k) 415/240 (a)<br />
240 (k)<br />
415/240 (a)<br />
Pakistan 50 230 (k) 400/230 (a)<br />
230 (k)<br />
400/230 (a)<br />
Papua New Guinea 50 ± 2 240 (k) 415/240 (a) 22,000<br />
240 (k) 11,000<br />
415/240 (a)<br />
Paraguay 50 ± 0.5 220 (k) 380/220 (a) 22,000<br />
220 (k) 380/220 (a)<br />
Philippines (Rep <strong>of</strong> the) 60 ± 0.16 110/220 (j) 13,800 13,800<br />
4,160 4,160<br />
2,400 2,400<br />
110/220 (h) 440 (b)<br />
110/220 (h)<br />
Poland 50 ± 0.1 230 (k) 400/230 (a) 1,000<br />
690/400<br />
400/230 (a)<br />
Portugal 50 ± 1 380/220 (a) 15,000 15,000<br />
220 (k) 5,000 5,000<br />
380/220 (a)<br />
220 (k)<br />
380/220 (a)<br />
Qatar 50 ± 0.1 415/240 (k) 415/240 (a) 11,000<br />
415/240 (a)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
C<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
C6<br />
C - Connecion to the LV public<br />
distribution network<br />
Fig. C1 : Voltage <strong>of</strong> local LV network and their associated circuit diagrams (continued on next page)<br />
Low-voltage utility distribution<br />
networks<br />
Country Frequency & tolerance Domestic (V) Commercial (V) Industrial (V)<br />
(Hz & %)<br />
Romania 50 ± 0.5 220 (k) 220/380 (a) 20,000<br />
220/380 (a) 10,000<br />
6,000<br />
220/380 (a)<br />
Russia 50 ± 0.2 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
Rwanda 50 ± 1 220 (k) 380/220 (a) 15,000<br />
6,600<br />
380/220 (a)<br />
Saint Lucia 50 ± 3 240 (k) 415/240 (a) 11,000<br />
415/240 (a)<br />
Samoa 400/230<br />
San Marino 50 ± 1 230/220 380 15,000<br />
380<br />
Saudi Arabia 60 220/127 (a) 220/127 (a) 11,000<br />
380/220 (a) 7,200<br />
380/220 (a)<br />
The Solomon Islands 50 ± 2 240 415/240 415/240<br />
Senegal 50 ± 5 220 (a) 380/220 (a) 90,000<br />
127 (k) 220/127 (k) 30,000<br />
6,600<br />
Serbia and Montenegro 50 380/220 (a) 380/220 (a) 10,000<br />
220 (k) 220 (k) 6,600<br />
380/220 (a)<br />
Seychelles 50 ± 1 400/230 (a) 400/230 (a) 11,000<br />
400/230 (a)<br />
Sierra Leone 50 ± 5 230 (k) 400/230 (a) 11,000<br />
230 (k) 400<br />
Singapore 50 400/230 (a) 400/230 (a) 22,000<br />
230 (k) 6,600<br />
400/230 (a)<br />
Slovakia 50 ± 0.5 230 230 230/400<br />
Slovenia 50 ± 0.1 220 (k) 380/220 (a) 10,000<br />
6,600<br />
380/220 (a)<br />
Somalia 50 230 (k) 440/220 (j) 440/220 (g)<br />
220 (k) 220/110 (j) 220/110 (g)<br />
110 (k) 230 (k)<br />
South Africa 50 ± 2.5 433/250 (a) 11,000 11,000<br />
400/230 (a) 6,600 6,600<br />
380/220 (a) 3,300 3,300<br />
220 (k) 433/250 (a) 500 (b)<br />
400/230 (a) 380/220 (a)<br />
380/220 (a)<br />
Spain 50 ± 3 380/220 (a) (e) 380/220 (a) 15,000<br />
220 (k) 220/127 (a) (e) 11,000<br />
220/127 (a) 380/220 (a)<br />
127 (k)<br />
Sri Lanka 50 ± 2 230 (k) 400/230 (a) 11,000<br />
230 (k) 400/230 (a)<br />
Sudan 50 240 (k) 415/240 (a) 415/240 (a)<br />
240 (k)<br />
Swaziland 50 ± 2.5 230 (k) 400/230 (a) 11,000<br />
230 (k) 400/230 (a)<br />
Sweden 50 ± 0.5 400/230 (a) 400/230 (a) 6,000<br />
230 (k) 230 (k) 400/230 (a)<br />
Switzerland 50 ± 2 400/230 (a) 400/230 (a) 20,000<br />
10,000<br />
3,000<br />
1,000<br />
690/500<br />
Syria 50 220 (k) 380/220 (a) 380/220 (a)<br />
115 (k) 220 (k)<br />
200/115 (a)<br />
Tadzhikistan 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k)<br />
220/127 (a)<br />
127 (k)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
C - Connecion to the LV public<br />
distribution network<br />
Fig. C1 : Voltage <strong>of</strong> local LV network and their associated circuit diagrams (continued on next page)<br />
Low-voltage utility distribution<br />
networks<br />
Country Frequency & tolerance<br />
(Hz & %)<br />
Domestic (V) Commercial (V) Industrial (V)<br />
Tanzania 50 400/230 (a) 400/230 (a) 11,000<br />
400/230 (a)<br />
Thailand 50 220 (k) 380/220 (a)<br />
220 (k)<br />
380/220 (a)<br />
Togo 50 220 (k) 380/220 (a) 20,000<br />
5,500<br />
380/220 (a)<br />
Tunisia 50 ± 2 380/220 (a) 380/220 (a) 30,000<br />
220 (k) 220 (k) 15,000<br />
10,000<br />
380/220 (a)<br />
Turkmenistan 50 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k)<br />
220/127 (a)<br />
127 (k)<br />
220 (k)<br />
Turkey 50 ± 1 380/220 (a) 380/220 (a) 15,000<br />
6,300<br />
380/220 (a)<br />
Uganda + 0.1 240 (k) 415/240 (a) 11,000<br />
415/240 (a)<br />
Ukraine + 0.2 / - 1.5 380/220 (a) 380/220 (a) 380/220 (a)<br />
220 (k) 220 (k) 220 (k)<br />
United Arab Emirates 50 ± 1 220 (k) 415/240 (a) 6,600<br />
380/220 (a) 415/210 (a)<br />
220 (k) 380/220 (a)<br />
United Kingdom 50 ± 1 230 (k) 400/230 (a) 22,000<br />
(except Northern 11,000<br />
Ireland) 6,600<br />
3,300<br />
400/230 (a)<br />
United Kingdom 50 ± 0.4 230 (k) 400/230 (a) 400/230 (a)<br />
(Including Northern<br />
Ireland)<br />
220 (k) 380/220 (a) 380/220 (a)<br />
United States <strong>of</strong> 60 ± 0.06 120/240 (j) 265/460 (a) 14,400<br />
America 120/208 (a) 120/240 (j) 7,200<br />
Charlotte 120/208 (a) 2,400<br />
(North Carolina) 575 (f)<br />
460 (f)<br />
240 (f)<br />
265/460 (a)<br />
120/240 (j)<br />
120/208 (a)<br />
United States <strong>of</strong> 60 ± 0.2 120/240 (j) 480 (f) 13,200<br />
America 120/208 (a) 120/240 (h) 4,800<br />
Detroit (Michigan) 120/208 (a) 4,160<br />
480 (f)<br />
120/240 (h)<br />
120/208 (a)<br />
United States <strong>of</strong> 60 ± 0.2 120/240 (j) 4,800 4,800<br />
America<br />
Los Angeles (California)<br />
120/240 (g) 120/240 (g)<br />
United States <strong>of</strong> 60 ± 0.3 120/240 (j) 120/240 (j) 13,200<br />
America 120/208 (a) 120/240 (h) 2,400<br />
Miami (Florida) 120/208 (a) 480/277 (a)<br />
120/240 (h)<br />
United States <strong>of</strong> 60 120/240 (j) 120/240 (j) 12,470<br />
America New York 120/208 (a) 120/208 (a) 4,160<br />
(New York) 240 (f) 277/480 (a)<br />
480 (f)<br />
United States <strong>of</strong> 60 ± 0.03 120/240 (j) 265/460 (a) 13,200<br />
America 120/240 (j) 11,500<br />
Pittsburg 120/208 (a) 2,400<br />
(Pennsylvania) 460 (f) 265/460 (a)<br />
230 (f) 120/208 (a)<br />
460 (f)<br />
230 (f)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
C<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
C<br />
C - Connecion to the LV public<br />
distribution network<br />
Country Frequency & tolerance Domestic (V) Commercial (V) Industrial (V)<br />
(Hz & %)<br />
United States <strong>of</strong> 60 120/240 (j) 227/480 (a) 19,900<br />
America 120/240 (j) 12,000<br />
Portland (Oregon) 120/208 (a) 7,200<br />
480 (f) 2,400<br />
240 (f) 277/480 (a)<br />
120/208 (a)<br />
480 (f)<br />
240 (f)<br />
United States <strong>of</strong> 60 ± 0.08 120/240 (j) 277/480 (a) 20,800<br />
America 120/240 (j) 12,000<br />
San Francisco 4,160<br />
(California) 277/480 (a)<br />
120/240 (g)<br />
United States <strong>of</strong> 60 ± 0.08 120/240 (j) 277/480 (c) 12,470<br />
America 120/208 (a) 120/240(h) 7,200<br />
Toledo (Ohio) 120/208 (j) 4,800<br />
4,160<br />
480 (f)<br />
277/480 (a)<br />
120/208 (a)<br />
Uruguay 50 ± 1 220 (b) (k) 220 (b) (k) 15,000<br />
6,000<br />
220 (b)<br />
Vietnam 50 ± 0.1 220 (k) 380/220 (a) 35,000<br />
15,000<br />
10,000<br />
6,000<br />
Yemen 50 250 (k) 440/250 (a) 440/250 (a)<br />
Zambia 50 ± 2.5 220 (k) 380/220 (a) 380 (a)<br />
Zimbabwe 50 225 (k) 390/225 (a) 11,000<br />
390/225 (a)<br />
Circuit diagrams<br />
(a) Three-phase star;<br />
Four-wire:<br />
Earthed neutral<br />
(f) Three-phase delta:<br />
Three-wire<br />
(j) Single-phase;<br />
Three-wire:<br />
Earthed mid point<br />
(b) Three-phase star:<br />
Three-wire<br />
(g) Three-phase delta;<br />
Four-wire:<br />
Earthed mid point <strong>of</strong><br />
one phase<br />
(k) Single-phase;<br />
Two-wire:<br />
Earthed end <strong>of</strong> phase<br />
(l) Single-phase;<br />
Two-wire<br />
Unearthed<br />
Fig. C1 : Voltage <strong>of</strong> local LV network and their associated circuit diagrams (concluded)<br />
Low-voltage utility distribution<br />
networks<br />
(c) Three-phase star;<br />
Three-wire:<br />
Earthed neutral<br />
(h) Three-phase open delta;<br />
Four-wire:<br />
Earthed mid point <strong>of</strong> one<br />
phase<br />
V Vk<br />
(m) Single-wire:<br />
Earthed return (swer)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
(d) Three-phase star;<br />
Four-wire:<br />
Non-earthed neutral<br />
(e) Two-phase star;<br />
Three-wire<br />
Earthed neutral<br />
(i) Three-phase<br />
open delta:<br />
Earthed junction<br />
<strong>of</strong> phases<br />
(n) DC:<br />
Three-wire:<br />
Unearthed
C - Connecion to the LV public<br />
distribution network<br />
(1) The Figure C2 values shown are indicative only, being<br />
(arbitrarily) based on 60 A maximum service currents for the<br />
first three systems, since smaller voltage drops are allowed at<br />
these lower voltages, for a given percentage statutory limit.<br />
The second group <strong>of</strong> systems is (again, arbitrarily) based on a<br />
maximum permitted service current <strong>of</strong> 120 A.<br />
Low-voltage utility distribution<br />
networks<br />
Residential and commercial consumers<br />
The function <strong>of</strong> a LV “mains” distributor is to provide service connections<br />
(underground cable or overhead line) to a number <strong>of</strong> consumers along its route.<br />
The current-rating requirements <strong>of</strong> distributors are estimated from the number <strong>of</strong><br />
consumers to be connected and an average demand per consumer.<br />
The two principal limiting parameters <strong>of</strong> a distributor are:<br />
b The maximum current which it is capable <strong>of</strong> carrying indefinitely, and<br />
b The maximum length <strong>of</strong> cable which, when carrying its maximum current, will not<br />
exceed the statutory voltage-drop limit<br />
These constraints mean that the magnitude <strong>of</strong> loads which utilities are willing to<br />
connect to their LV distribution mains, is necessarily restricted.<br />
For the range <strong>of</strong> LV systems mentioned in the second paragraph <strong>of</strong> this sub-clause<br />
(1.1) viz: 120 V single phase to 240/415 V 3-phase, typical maximum permitted loads<br />
connected to a LV distributor might (1) be (see Fig. C2):<br />
System Assumed max. permitted current kVA<br />
per consumer service<br />
120 V 1-phase 2-wire 60 A 7.2<br />
120/240 V 1-phase 3-wire 60 A 14.4<br />
120/208 V 3-phase 4-wire 60 A 22<br />
220/380 V 3-phase 4-wire 120 A 80<br />
230/400 V 3-phase 4-wire 120 A 83<br />
240/415 V 3-phase 4-wire 120 A 86<br />
Fig. C2 : Typical maximum permitted loads connected to a LV distributor<br />
Practices vary considerably from one power supply organization to another, and no<br />
“standardized” values can be given.<br />
Factors to be considered include:<br />
b The size <strong>of</strong> an existing distribution network to which the new load is to be connected<br />
b The total load already connected to the distribution network<br />
b The location along the distribution network <strong>of</strong> the proposed new load, i.e. close to<br />
the substation, or near the remote end <strong>of</strong> the distribution network, etc<br />
In short, each case must be examined individually.<br />
The load levels listed above are adequate for all normal residential consumers, and<br />
will be sufficient for the <strong>installation</strong>s <strong>of</strong> many administrative, commercial and similar<br />
buildings.<br />
Medium-size and small industrial consumers (with dedicated<br />
LV lines direct from a utility supply MV/LV substation)<br />
Medium and small industrial consumers can also be satisfactorily supplied at lowvoltage.<br />
For loads which exceed the maximum permitted limit for a service from a distributor,<br />
a dedicated cable can usually be provided from the LV distribution fuse- (or switch-)<br />
board, in the power utility substation.<br />
<strong>General</strong>y, the upper load limit which can be supplied by this means is restricted only<br />
by the available spare transformer capacity in the substation.<br />
In practice, however:<br />
b Large loads (e.g. > 300 kVA) require correspondingly large cables, so that,<br />
unless the load centre is close to the substation, this method can be economically<br />
unfavourable<br />
b Many utilities prefer to supply loads exceeding 200 kVA (this figure varies with<br />
different suppliers) at medium voltage<br />
For these reasons, dedicated supply lines at LV are generally applied (at 220/380 V<br />
to 240/415 V) to a load range <strong>of</strong> 80 kVA to 250 kVA.<br />
Consumers normally supplied at low voltage include:<br />
b Residential dwellings<br />
b Shops and commercial buildings<br />
b Small factories, workshops and filling stations<br />
b Restaurants<br />
b Farms, etc<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
C<br />
© Schneider Electric - all rights reserved
C 0<br />
© Schneider Electric - all rights reserved<br />
C - Connecion to the LV public<br />
distribution network<br />
In cities and large towns, standardized<br />
LV distribution cables form a network through<br />
link boxes. Some links are removed, so that<br />
each (fused) distributor leaving a substation<br />
forms a branched open-ended radial system,<br />
as shown in Figure C3<br />
Low-voltage utility distribution<br />
networks<br />
.2 LV distribution networks<br />
In European countries the standard 3-phase 4-wire distribution voltage level is<br />
230/400 V. Many countries are currently converting their LV systems to the latest IEC<br />
standard <strong>of</strong> 230/400 V nominal (IEC 60038). Medium<br />
to large-sized towns and cities have underground cable distribution systems.<br />
MV/LV distribution substations, mutually spaced at approximately 500-600 metres,<br />
are typically equipped with:<br />
b A 3-or 4-way MV switchboard, <strong>of</strong>ten made up <strong>of</strong> incoming and outgoing loadbreak<br />
switches forming part <strong>of</strong> a ring main, and one or two MV circuit-breakers or<br />
combined fuse/ load-break switches for the transformer circuits<br />
b One or two 1,000 kVA MV/LV transformers<br />
b One or two (coupled) 6-or 8-way LV 3-phase 4-wire distribution fuse boards, or<br />
moulded-case circuit-breaker boards, control and protect outgoing 4-core distribution<br />
cables, generally referred to as “distributors”<br />
The output from a transformer is connected to the LV busbars via a load-break<br />
switch, or simply through isolating links.<br />
In densely-loaded areas, a standard size <strong>of</strong> distributor is laid to form a network,<br />
with (generally) one cable along each pavement and 4-way link boxes located in<br />
manholes at street corners, where two cables cross.<br />
Recent trends are towards weather-pro<strong>of</strong> cabinets above ground level, either against<br />
a wall, or where possible, flush-mounted in the wall.<br />
Links are inserted in such a way that distributors form radial circuits from the<br />
substation with open-ended branches (see Fig. C3). Where a link box unites a<br />
distributor from one substation with that from a neighbouring substation, the phase<br />
links are omitted or replaced by fuses, but the neutral link remains in place.<br />
Service<br />
cable<br />
Fig. C3 : Showing one <strong>of</strong> several ways in which a LV distribution network may be arranged for<br />
radial branched-distributor operation, by removing (phase) links<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
HV/LV<br />
substation<br />
4-way<br />
link box<br />
Phase links<br />
removed
C - Connecion to the LV public<br />
distribution network<br />
In less-densely loaded urban areas a moreeconomic<br />
system <strong>of</strong> tapered radial distribution<br />
is commonly used, in which conductors <strong>of</strong><br />
reduced size are installed as the distance from<br />
a substation increases<br />
Improved methods using insulated twisted<br />
conductors to form a pole mounted aerial cable<br />
are now standard practice in many countries<br />
In Europe, each utility-supply distribution<br />
substation is able to supply at LV an area<br />
corresponding to a radius <strong>of</strong> approximately<br />
300 metres from the substation.<br />
North and Central American systems <strong>of</strong><br />
distribution consist <strong>of</strong> a MV network from which<br />
numerous (small) MV/LV transformers each<br />
supply one or several consumers, by direct<br />
service cable (or line) from the transformer<br />
location<br />
Service components and metering equipment<br />
were formerly installed inside a consumer’s<br />
building. The modern tendency is to locate<br />
these items outside in a weatherpro<strong>of</strong> cabinet<br />
Low-voltage utility distribution<br />
networks<br />
This arrangement provides a very flexible system in which a complete substation can<br />
be taken out <strong>of</strong> service, while the area normally supplied from it is fed from link boxes<br />
<strong>of</strong> the surrounding substations.<br />
Moreover, short lengths <strong>of</strong> distributor (between two link boxes) can be isolated for<br />
fault-location and repair.<br />
Where the load density requires it, the substations are more closely spaced, and<br />
transformers up to 1,500 kVA are sometimes necessary.<br />
Other forms <strong>of</strong> urban LV network, based on free-standing LV distribution pillars,<br />
placed above ground at strategic points in the network, are widely used in areas <strong>of</strong><br />
lower load density. This scheme exploits the principle <strong>of</strong> tapered radial distributors in<br />
which the distribution cable conductor size is reduced as the number <strong>of</strong> consumers<br />
downstream diminish with distance from the substation.<br />
In this scheme a number <strong>of</strong> large-sectioned LV radial feeders from the distribution<br />
board in the substation supply the busbars <strong>of</strong> a distribution pillar, from which smaller<br />
distributors supply consumers immediately surrounding the pillar.<br />
Distribution in market towns, villages and rural areas generally has, for many years,<br />
been based on bare copper conductors supported on wooden, concrete or steel<br />
poles, and supplied from pole-mounted or ground-mounted transformers.<br />
In recent years, LV insulated conductors, twisted to form a two-core or 4-core self<br />
supporting cable for overhead use, have been developed, and are considered to be<br />
safer and visually more acceptable than bare copper lines.<br />
This is particularly so when the conductors are fixed to walls (e.g. under-eaves<br />
wiring) where they are hardly noticeable.<br />
As a matter <strong>of</strong> interest, similar principles have been applied at higher voltages, and<br />
self supporting “bundled” insulated conductors for MV overhead <strong>installation</strong>s are now<br />
available for operation at 24 kV.<br />
Where more than one substation supplies a village, arrangements are made at poles<br />
on which the LV lines from different substations meet, to interconnect corresponding<br />
phases.<br />
North and Central American practice differs fundamentally from that in Europe, in<br />
that LV networks are practically nonexistent, and 3-phase supplies to premises in<br />
residential areas are rare.<br />
The distribution is effectively carried out at medium voltage in a way, which again<br />
differs from standard European practices. The MV system is, in fact, a 3-phase<br />
4-wire system from which single-phase distribution networks (phase and neutral<br />
conductors) supply numerous single-phase transformers, the secondary windings<br />
<strong>of</strong> which are centre-tapped to produce 120/240 V single-phase 3-wire supplies.<br />
The central conductors provide the LV neutrals, which, together with the MV neutral<br />
conductors, are solidly earthed at intervals along their lengths.<br />
Each MV/LV transformer normally supplies one or several premises directly from the<br />
transformer position by radial service cable(s) or by overhead line(s).<br />
Many other systems exist in these countries, but the one described appears to be<br />
the most common.<br />
Figure C (next page) shows the main features <strong>of</strong> the two systems.<br />
. The consumer-service connection<br />
In the past, an underground cable service or the wall-mounted insulated conductors<br />
from an overhead line service, invariably terminated inside the consumer’s premises,<br />
where the cable-end sealing box, the utility fuses (inaccessible to the consumer) and<br />
meters were installed.<br />
A more recent trend is (as far as possible) to locate these service components in a<br />
weatherpro<strong>of</strong> housing outside the building.<br />
The utility/consumer interface is <strong>of</strong>ten at the outgoing terminals <strong>of</strong> the meter(s) or,<br />
in some cases, at the outgoing terminals <strong>of</strong> the <strong>installation</strong> main circuit-breaker<br />
(depending on local practices) to which connection is made by utility staff, following a<br />
satisfactory test and inspection <strong>of</strong> the <strong>installation</strong>.<br />
A typical arrangement is shown in Figure C (next page).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
C<br />
© Schneider Electric - all rights reserved
C 2<br />
© Schneider Electric - all rights reserved<br />
C - Connecion to the LV public<br />
distribution network<br />
1 2 3<br />
1<br />
2<br />
13.8 kV / 2.4-4.16 kV<br />
Fig. C4 : Widely-used American and European-type systems<br />
N<br />
Each MV/LV transformer shown<br />
represents many similar units<br />
2.4 kV / 120-240 V<br />
1 ph - 3 wire<br />
distribution<br />
transformer<br />
N<br />
N<br />
Low-voltage utility distribution<br />
networks<br />
2<br />
3<br />
N<br />
1<br />
N<br />
2<br />
N<br />
HV (1)<br />
MV (2)<br />
Fig. C5 : Typical service arrangement for TT-earthed systems<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
For primary voltages > 72.5 kV<br />
(see note) primary<br />
winding may be:<br />
- Delta<br />
- Earthed star<br />
- Earthed zigzag<br />
Depending on the country concerned<br />
Tertiary delta normally<br />
(not always) used if the<br />
primary winding is not delta<br />
1 ph MV / 230 V<br />
service transformer<br />
to isolated consumer(s)<br />
(rural supplies)<br />
Resistor replaced<br />
by a Petersen<br />
coil on O/H line<br />
systems in some<br />
countries<br />
Ph N<br />
1 2 3 N<br />
N<br />
N 1 2 3<br />
Main 3 ph and neutral<br />
MV distributor<br />
LV distribution network<br />
(1) 132 kV for example<br />
(2) 11 kV for example<br />
Note: At primary voltages greater than 72.5 kV in bulk-supply substations, it is common practice in some European countries<br />
to use an earthed-star primary winding and a delta secondary winding. The neutral point on the secondary side is then<br />
provided by a zigzag earthing reactor, the star point <strong>of</strong> which is connected to earth through a resistor.<br />
Frequently, the earthing reactor has a secondary winding to provide LV 3-phase supplies for the substation. It is then referred<br />
to as an “earthing transformer”.<br />
CB<br />
M<br />
F<br />
A<br />
}<br />
3 ph<br />
MV / 230/400 V<br />
4-wire distribution<br />
transformer
C - Connecion to the LV public<br />
distribution network<br />
LV consumers are normally supplied according<br />
to the TN or TT system, as described in<br />
chapters F and G. The <strong>installation</strong> main circuitbreaker<br />
for a TT supply must include a residual<br />
current earth-leakage protective device. For a<br />
TN service, overcurrent protection by circuitbreaker<br />
or switch-fuse is required<br />
Low-voltage utility distribution<br />
networks<br />
A MCCB -moulded case circuit-breaker- which incorporates a sensitive residualcurrent<br />
earth-fault protective feature is mandatory at the origin <strong>of</strong> any LV <strong>installation</strong><br />
forming part <strong>of</strong> a TT earthing system. The reason for this feature and related<br />
leakage-current tripping levels are discussed in Clause 3 <strong>of</strong> <strong>Chapter</strong> G.<br />
A further reason for this MCCB is that the consumer cannot exceed his (contractual)<br />
declared maximum load, since the overload trip setting, which is sealed by the<br />
supply authority, will cut <strong>of</strong>f supply above the declared value. Closing and tripping <strong>of</strong><br />
the MCCB is freely available to the consumer, so that if the MCCB is inadvertently<br />
tripped on overload, or due to an appliance fault, supplies can be quickly restored<br />
following correction <strong>of</strong> the anomaly.<br />
In view <strong>of</strong> the inconvenience to both the meter reader and consumer, the location <strong>of</strong><br />
meters is nowadays generally outside the premises, either:<br />
b In a free-standing pillar-type housing as shown in Figures C6 and C<br />
b In a space inside a building, but with cable termination and supply authority’s fuses<br />
located in a flush-mounted weatherpro<strong>of</strong> cabinet accessible from the public way, as<br />
shown in Figure C next page<br />
b For private residential consumers, the equipment shown in the cabinet in<br />
Figure C5 is installed in a weatherpro<strong>of</strong> cabinet mounted vertically on a metal<br />
frame in the front garden, or flush-mounted in the boundary wall, and accessible to<br />
authorized personnel from the pavement. Figure C (next page) shows the general<br />
arrangement, in which removable fuse links provide the means <strong>of</strong> isolation<br />
Fig. C6 : Typical rural-type <strong>installation</strong><br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
F<br />
A<br />
M<br />
CB<br />
In this kind <strong>of</strong> <strong>installation</strong> it is <strong>of</strong>ten necessary to place the main <strong>installation</strong> circuitbreaker<br />
some distance from the point <strong>of</strong> utilization, e.g. saw-mills, pumping stations,<br />
etc.<br />
The main <strong>installation</strong> CB is located in the consumer’s premises in cases where it is<br />
set to trip if the declared kVA load demand is exceeded.<br />
CB<br />
Fig. C7 : Semi-urban <strong>installation</strong>s (shopping precincts, etc.)<br />
M<br />
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C - Connecion to the LV public<br />
distribution network<br />
Low-voltage utility distribution<br />
networks<br />
The service cable terminates in a flushmounted wall cabinet which contains the<br />
isolating fuse links, accessible from the public way. This method is preferred for<br />
esthetic reasons, when the consumer can provide a suitable metering and mainswitch<br />
location.<br />
Fig. C8 : Town centre <strong>installation</strong>s<br />
Service cable<br />
Isolation by<br />
fuse links<br />
Meter cabinet<br />
Fig. C9 : Typical LV service arrangement for residential consumers<br />
In the field <strong>of</strong> electronic metering, techniques have developed which make their<br />
use attractive by utilities either for electricity metering and for billing purposes, the<br />
liberalisation <strong>of</strong> the electricity market having increased the needs for more data<br />
collection to be returned from the meters. For example electronic metering can also<br />
help utilities to understand their customers’ consumption pr<strong>of</strong>iles. In the same way,<br />
they will be useful for more and more power line communication and radio-frequency<br />
applications as well.<br />
In this area, prepayment systems are also more and more employed when<br />
economically justified. They are based on the fact that for instance consumers having<br />
made their payment at vending stations, generate tokens to pass the information<br />
concerning this payment on to the meters. For these systems the key issues are<br />
security and inter-operability which seem to have been addressed successfully<br />
now. The attractiveness <strong>of</strong> these systems is due to the fact they not only replace the<br />
meters but also the billing systems, the reading <strong>of</strong> meters and the administration <strong>of</strong><br />
the revenue collection.<br />
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Installation
C - Connecion to the LV public<br />
distribution network<br />
An adequate level <strong>of</strong> voltage at the consumers<br />
supply-service terminals is essential for<br />
satisfactory operation <strong>of</strong> equipment and<br />
appliances. Practical values <strong>of</strong> current, and<br />
resulting voltage drops in a typical LV system,<br />
show the importance <strong>of</strong> maintaining a high<br />
Power Factor as a means <strong>of</strong> reducing voltage<br />
drop.<br />
(1) Transformers <strong>design</strong>ed for the 230/400 V IEC standard<br />
will have a no-load output <strong>of</strong> 420 V, i.e. 105% <strong>of</strong> the nominal<br />
voltage<br />
Low-voltage utility distribution<br />
networks<br />
. Quality <strong>of</strong> supply voltage<br />
The quality <strong>of</strong> the LV network supply voltage in its widest sense implies:<br />
b Compliance with statutory limits <strong>of</strong> magnitude and frequency<br />
b Freedom from continual fluctuation within those limits<br />
b Uninterrupted power supply, except for scheduled maintenance shutdowns, or as a<br />
result <strong>of</strong> system faults or other emergencies<br />
b Preservation <strong>of</strong> a near-sinusoidal wave form<br />
In this Sub-clause the maintenance <strong>of</strong> voltage magnitude only will be discussed.<br />
In most countries, power-supply authorities have a statutory obligation to maintain<br />
the level <strong>of</strong> voltage at the service position <strong>of</strong> consumers within the limits <strong>of</strong> ± 5% (or<br />
in some cases ± 6% or more-see table C1) <strong>of</strong> the declared nominal value.<br />
Again, IEC and most national standards recommend that LV appliances be <strong>design</strong>ed<br />
and tested to perform satisfactorily within the limits <strong>of</strong> ± 10% <strong>of</strong> nominal voltage. This<br />
leaves a margin, under the worst conditions (<strong>of</strong> minus 5% at the service position, for<br />
example) <strong>of</strong> 5% allowable voltage drop in the <strong>installation</strong> wiring.<br />
The voltage drops in a typical distribution system occur as follows: the voltage at<br />
the MV terminals <strong>of</strong> a MV/LV transformer is normally maintained within a ± 2% band<br />
by the action <strong>of</strong> automatic onload tapchangers <strong>of</strong> the transformers at bulk-supply<br />
substations, which feed the MV network from a higher-voltage subtransmission<br />
system.<br />
If the MV/LV transformer is in a location close to a bulk-supply substation, the ± 2%<br />
voltage band may be centered on a voltage level which is higher than the nominal<br />
MV value. For example, the voltage could be 20.5 kV ± 2% on a 20 kV system. In this<br />
case, the MV/LV distribution transformer should have its MV <strong>of</strong>f-circuit tapping switch<br />
selected to the + 2.5% tap position.<br />
Conversely, at locations remote from bulk supply substations a value <strong>of</strong> 19.5 kV ±<br />
2% is possible, in which case the <strong>of</strong>f-circuit tapping switch should be selected to the<br />
- 5% position.<br />
The different levels <strong>of</strong> voltage in a system are normal, and depend on the system<br />
powerflow pattern. Moreover, these voltage differences are the reason for the term<br />
“nominal” when referring to the system voltage.<br />
Practical application<br />
With the MV/LV transformer correctly selected at its <strong>of</strong>f-circuit tapping switch, an<br />
unloaded transformer output voltage will be held within a band <strong>of</strong> ± 2% <strong>of</strong> its no-load<br />
voltage output.<br />
To ensure that the transformer can maintain the necessary voltage level when fully<br />
loaded, the output voltage at no-load must be as high as possible without exceeding<br />
the upper + 5% limit (adopted for this example). In present-day practice, the winding<br />
ratios generally give an output voltage <strong>of</strong> about 104% at no-load (1) , when nominal<br />
voltage is applied at MV, or is corrected by the tapping switch, as described above.<br />
This would result in a voltage band <strong>of</strong> 102% to 106% in the present case.<br />
A typical LV distribution transformer has a short-circuit reactance voltage <strong>of</strong> 5%. If it<br />
is assumed that its resistance voltage is one tenth <strong>of</strong> this value, then the voltage drop<br />
within the transformer when supplying full load at 0.8 power factor lagging, will be:<br />
V% drop = R% cos ϕ + X% sin ϕ<br />
= 0.5 x 0.8 + 5 x 0.6<br />
= 0.4 + 3 = 3.4%<br />
The voltage band at the output terminals <strong>of</strong> the fully-loaded transformer will therefore<br />
be (102 - 3.4) = 98.6% to (106 - 3.4) = 102.6%.<br />
The maximum allowable voltage drop along a distributor is therefore 98.6 - 95 = 3.6%.<br />
This means, in practical terms, that a medium-sized 230/400 V 3-phase 4-wire<br />
distribution cable <strong>of</strong> 240 mm 2 copper conductors would be able to supply a total load<br />
<strong>of</strong> 292 kVA at 0.8 PF lagging, distributed evenly over 306 metres <strong>of</strong> the distributor.<br />
Alternatively, the same load at the premises <strong>of</strong> a single consumer could be supplied<br />
at a distance <strong>of</strong> 153 metres from the transformer, for the same volt-drop, and so on...<br />
As a matter <strong>of</strong> interest, the maximum rating <strong>of</strong> the cable, based on calculations<br />
derived from IEC 60287 (1982) is 290 kVA, and so the 3.6% voltage margin is not<br />
unduly restrictive, i.e. the cable can be fully loaded for distances normally required in<br />
LV distribution systems.<br />
Furthermore, 0.8 PF lagging is appropriate to industrial loads. In mixed semiindustrial<br />
areas 0.85 is a more common value, while 0.9 is generally used for<br />
calculations concerning residential areas, so that the volt-drop noted above may be<br />
considered as a “worst case” example.<br />
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C - Connecion to the LV public<br />
distribution network<br />
(1) Ripple control is a system <strong>of</strong> signalling in which a voice<br />
frequency current (commonly at 175 Hz) is injected into the<br />
LV mains at appropriate substations. The signal is injected<br />
as coded impulses, and relays which are tuned to the signal<br />
frequency and which recognize the particular code will operate<br />
to initiate a required function. In this way, up to 960 discrete<br />
control signals are available.<br />
2 Tariffs and metering<br />
No attempt will be made in this guide to discuss particular tariffs, since there appears<br />
to be as many different tariff structures around the world as there are utilities.<br />
Some tariffs are very complicated in detail but certain elements are basic to all <strong>of</strong><br />
them and are aimed at encouraging consumers to manage their power consumption<br />
in a way which reduces the cost <strong>of</strong> generation, transmission and distribution.<br />
The two predominant ways in which the cost <strong>of</strong> supplying power to consumers can<br />
be reduced, are:<br />
b Reduction <strong>of</strong> power losses in the generation, transmission and distribution <strong>of</strong><br />
<strong>electrical</strong> energy. In principle the lowest losses in a power system are attained when<br />
all parts <strong>of</strong> the system operate at unity power factor<br />
b Reduction <strong>of</strong> the peak power demand, while increasing the demand at low-load<br />
periods, thereby exploiting the generating plant more fully, and minimizing plant<br />
redundancy<br />
Reduction <strong>of</strong> losses<br />
Although the ideal condition noted in the first possibility mentioned above cannot<br />
be realized in practice, many tariff structures are based partly on kVA demand, as<br />
well as on kWh consumed. Since, for a given kW loading, the minimum value <strong>of</strong> kVA<br />
occurs at unity power factor, the consumer can minimize billing costs by taking steps<br />
to improve the power factor <strong>of</strong> the load (as discussed in <strong>Chapter</strong> L). The kVA demand<br />
generally used for tariff purposes is the maximum average kVA demand occurring<br />
during each billing period, and is based on average kVA demands, over fixed periods<br />
(generally 10, 30 or 60 minute periods) and selecting the highest <strong>of</strong> these values.<br />
The principle is described below in “principle <strong>of</strong> kVA maximum-demand metering”.<br />
Reduction <strong>of</strong> peak power demand<br />
The second aim, i.e. that <strong>of</strong> reducing peak power demands, while increasing demand<br />
at low-load periods, has resulted in tariffs which <strong>of</strong>fer substantial reduction in the cost<br />
<strong>of</strong> energy at:<br />
b Certain hours during the 24-hour day<br />
b Certain periods <strong>of</strong> the year<br />
The simplest example is that <strong>of</strong> a residential consumer with a storage-type water<br />
heater (or storage-type space heater, etc.). The meter has two digital registers, one<br />
<strong>of</strong> which operates during the day and the other (switched over by a timing device)<br />
operates during the night. A contactor, operated by the same timing device, closes<br />
the circuit <strong>of</strong> the water heater, the consumption <strong>of</strong> which is then indicated on the<br />
register to which the cheaper rate applies. The heater can be switched on and <strong>of</strong>f at<br />
any time during the day if required, but will then be metered at the normal rate. Large<br />
industrial consumers may have 3 or 4 rates which apply at different periods during<br />
a 24-hour interval, and a similar number for different periods <strong>of</strong> the year. In such<br />
schemes the ratio <strong>of</strong> cost per kWh during a period <strong>of</strong> peak demand for the year, and<br />
that for the lowest-load period <strong>of</strong> the year, may be as much as 10: 1.<br />
Meters<br />
It will be appreciated that high-quality instruments and devices are necessary to<br />
implement this kind <strong>of</strong> metering, when using classical electro-mechanical equipment.<br />
Recent developments in electronic metering and micro-processors, together with<br />
remote ripple-control (1) from an utility control centre (to change peak-period timing<br />
throughout the year, etc.) are now operational, and facilitate considerably the<br />
application <strong>of</strong> the principles discussed.<br />
In most countries, some tariffs, as noted above, are partly based on kVA demand,<br />
in addition to the kWh consumption, during the billing periods (<strong>of</strong>ten 3-monthly<br />
intervals). The maximum demand registered by the meter to be described, is, in fact,<br />
a maximum (i.e. the highest) average kVA demand registered for succeeding periods<br />
during the billing interval.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
C - Connecion to the LV public<br />
distribution network<br />
2 Tariffs and metering<br />
Figure C10 shows a typical kVA demand curve over a period <strong>of</strong> two hours divided<br />
into succeeding periods <strong>of</strong> 10 minutes. The meter measures the average value <strong>of</strong><br />
kVA during each <strong>of</strong> these 10 minute periods.<br />
kVA<br />
Fig. C10 : Maximum average value <strong>of</strong> kVA over an interval <strong>of</strong> 2 hours<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Maximum average value<br />
during the 2 hour interval<br />
Average values<br />
for 10 minute<br />
periods<br />
0 1 2 hrs<br />
Principle <strong>of</strong> kVA maximum demand metering<br />
A kVAh meter is similar in all essentials to a kWh meter but the current and voltage<br />
phase relationship has been modified so that it effectively measures kVAh (kilovolt-ampere-hours).<br />
Furthermore, instead <strong>of</strong> having a set <strong>of</strong> decade counter dials,<br />
as in the case <strong>of</strong> a conventional kWh meter, this instrument has a rotating pointer.<br />
When the pointer turns it is measuring kVAh and pushing a red indicator before it.<br />
At the end <strong>of</strong> 10 minutes the pointer will have moved part way round the dial (it is<br />
<strong>design</strong>ed so that it can never complete one revolution in 10 minutes) and is then<br />
<strong>electrical</strong>ly reset to the zero position, to start another 10 minute period. The red<br />
indicator remains at the position reached by the measuring pointer, and that position,<br />
corresponds to the number <strong>of</strong> kVAh (kilo-volt-ampere-hours) taken by the load in<br />
10 minutes. Instead <strong>of</strong> the dial being marked in kVAh at that point however it can be<br />
marked in units <strong>of</strong> average kVA. The following figures will clarify the matter.<br />
Supposing the point at which the red indicator reached corresponds to 5 kVAh.<br />
It is known that a varying amount <strong>of</strong> kVA <strong>of</strong> apparent power has been flowing for<br />
10 minutes, i.e. 1/6 hour.<br />
If now, the 5 kVAh is divided by the number <strong>of</strong> hours, then the average kVA for the<br />
period is obtained.<br />
In this case the average kVA for the period will be:<br />
5 x 1 = 5 x 6 = 30 kVA<br />
1<br />
6<br />
Every point around the dial will be similarly marked i.e. the figure for average kVA will<br />
be 6 times greater than the kVAh value at any given point. Similar reasoning can be<br />
applied to any other reset-time interval.<br />
At the end <strong>of</strong> the billing period, the red indicator will be at the maximum <strong>of</strong> all the<br />
average values occurring in the billing period.<br />
The red indicator will be reset to zero at the beginning <strong>of</strong> each billing period. Electromechanical<br />
meters <strong>of</strong> the kind described are rapidly being replaced by electronic<br />
instruments. The basic measuring principles on which these electronic meters<br />
depend however, are the same as those described above.<br />
t<br />
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4<br />
5<br />
6<br />
7<br />
8<br />
<strong>Chapter</strong> D<br />
MV & LV architecture selection<br />
guide<br />
Contents<br />
Stakes for the user D3<br />
Simplified architecture <strong>design</strong> process D4<br />
2.1 The architecture <strong>design</strong> D4<br />
2.2 The whole process D5<br />
Electrical <strong>installation</strong> characteristics D7<br />
3.1 Activity D7<br />
3.2 Site topology D7<br />
3.3 Layout latitude D7<br />
3.4 Service reliability D8<br />
3.5 Maintainability D8<br />
3.6 Installation flexibility D8<br />
3.7 Power demand D8<br />
3.8 Load distribution D9<br />
3.9 Power interruption sensitivity D9<br />
3.10 Disturbance sensitivity D9<br />
3.11 Disturbance capability <strong>of</strong> circuits D10<br />
3.12 Other considerations or constraints D10<br />
Technological characteristics D<br />
4.1 Environment, atmosphere D11<br />
4.2 Service Index D11<br />
4.3 Other considerations D12<br />
Architecture assessment criteria D 3<br />
5.1 On-site work time D13<br />
5.2 Environmental impact D13<br />
5.3 Preventive maintenance level D13<br />
5.4 Availability <strong>of</strong> <strong>electrical</strong> power supply D14<br />
Choice <strong>of</strong> architecture fundamentals D 5<br />
6.1 Connection to the upstream network D15<br />
6.2 MV circuit configuration D16<br />
6.3 Number and distribution <strong>of</strong> MV/LV transformation substations D17<br />
6.4 Number <strong>of</strong> MV/LV transformers D18<br />
6.5 MV back-up generator D18<br />
Choice <strong>of</strong> architecture details D 9<br />
7.1 Layout D19<br />
7.2 Centralized or distributed layout D20<br />
7.3 Presence <strong>of</strong> an Uninterruptible Power Supply (UPS) D22<br />
7.4 Configuration <strong>of</strong> LV circuits D22<br />
Choice <strong>of</strong> equiment D24<br />
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9<br />
0<br />
2<br />
Recommendations for architecture optimization D26<br />
9.1 On-site work time D26<br />
9.2 Environmental impact D26<br />
9.3 Preventive maintenance volume D28<br />
9.4 Electrical power availability D28<br />
Glossary D29<br />
ID-Spec s<strong>of</strong>tware D30<br />
Example: <strong>electrical</strong> <strong>installation</strong> in a printworks D3<br />
12.1 Brief description D31<br />
12.2 Installation characteristics D31<br />
12.3 Technological characteristics D31<br />
12.4 Architecture assessment criteria D32<br />
12.5 Choice <strong>of</strong> technogical solutions D34<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
D - MV & LV architecture selection guide<br />
Potential for<br />
optimization<br />
Fig. D1 : Optimization potential<br />
ID-Spec<br />
Preliminary<br />
<strong>design</strong><br />
Stakes for the user<br />
Choice <strong>of</strong> distribution architecture<br />
The choice <strong>of</strong> distribution architecture has a decisive impact on <strong>installation</strong><br />
performance throughout its lifecycle:<br />
b right from the construction phase, choices can greatly influence the <strong>installation</strong><br />
time, possibilities <strong>of</strong> work rate, required competencies <strong>of</strong> <strong>installation</strong> teams, etc.<br />
b there will also be an impact on performance during the operation phase in terms<br />
<strong>of</strong> quality and continuity <strong>of</strong> power supply to sensitive loads, power losses in power<br />
supply circuits,<br />
b and lastly, there will be an impact on the proportion <strong>of</strong> the <strong>installation</strong> that can be<br />
recycled in the end-<strong>of</strong>-life phase.<br />
The Electrical Distribution architecture <strong>of</strong> an <strong>installation</strong> involves the spatial<br />
configuration, the choice <strong>of</strong> power sources, the definition <strong>of</strong> different distribution<br />
levels, the single-line diagram and the choice <strong>of</strong> equipment.<br />
The choice <strong>of</strong> the best architecture is <strong>of</strong>ten expressed in terms <strong>of</strong> seeking a<br />
compromise between the various performance criteria that interest the customer who<br />
will use the <strong>installation</strong> at different phases in its lifecycle. The earlier we search for<br />
solutions, the more optimization possibilities exist (see Fig. D1).<br />
Ecodial<br />
Detailled<br />
<strong>design</strong><br />
Installation<br />
A successful search for an optimal solution is also strongly linked to the ability for<br />
exchange between the various players involved in <strong>design</strong>ing the various sections <strong>of</strong><br />
a project:<br />
b the architect who defines the organization <strong>of</strong> the building according to user<br />
requirements,<br />
b the <strong>design</strong>ers <strong>of</strong> different technical sections (lighting, heating, air conditioning,<br />
fluids, etc.),<br />
b the user’s representatives e.g. defining the process.<br />
The following paragraphs present the selection criteria as well as the architecture<br />
<strong>design</strong> process to meet the project performance criteria in the context <strong>of</strong> industrial<br />
and tertiary buildings (excluding large sites).<br />
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Exploitation<br />
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2 Simplified architecture <strong>design</strong><br />
process<br />
2.1 The architecture <strong>design</strong><br />
The architecture <strong>design</strong> considered in this document is positioned at the Draft<br />
Design stage. It generally covers the levels <strong>of</strong> MV/LV main distribution, LV power<br />
distribution, and exceptionally the terminal distribution level. (see Fig. D2).<br />
MV/LV main<br />
distribution<br />
LV power<br />
distribution<br />
LV terminal<br />
distribution<br />
The <strong>design</strong> <strong>of</strong> an <strong>electrical</strong> distribution architecture can be described by a 3-stage<br />
process, with iterative possibilities. This process is based on taking account <strong>of</strong> the<br />
<strong>installation</strong> characteristics and criteria to be satisfied.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
M M M M<br />
Fig. D2 : Example <strong>of</strong> single-line diagram
D - MV & LV architecture selection guide<br />
Data<br />
Step<br />
Deliverable<br />
See § 4<br />
Technological<br />
characteristics<br />
See § 5<br />
Assessment<br />
criteria<br />
Fig. D3 : Flow diagram for choosing the <strong>electrical</strong> distribution architecture<br />
2 Simplified architecture <strong>design</strong><br />
process<br />
2.2 The whole process<br />
The whole process is described briefly in the following paragraphs and illustrated on<br />
Figure D3.<br />
The process described in this document is not intended as the only solution. This<br />
document is a guide intended for the use <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong>ers.<br />
See § 3<br />
Installation<br />
characteristics<br />
See § 6<br />
Step 1<br />
Choice <strong>of</strong><br />
fundamentals<br />
Schematic<br />
diagram<br />
See § 7<br />
Step 2<br />
Choice <strong>of</strong><br />
architecturedetails<br />
See § 8<br />
Detailed<br />
diagram<br />
Step 3<br />
Choice <strong>of</strong><br />
equipment<br />
Techno.<br />
Solution<br />
ASSESSMENT<br />
Definitive<br />
solution<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
See § 9<br />
Optimisation<br />
recommendations<br />
Step 1: Choice <strong>of</strong> distribution architecture fundamentals<br />
This involves defining the general features <strong>of</strong> the <strong>electrical</strong> <strong>installation</strong>. It is based<br />
on taking account <strong>of</strong> macroscopic characteristics concerning the <strong>installation</strong> and its<br />
usage.<br />
These characteristics have an impact on the connection to the upstream network,<br />
MV circuits, the number <strong>of</strong> transformer substations, etc.<br />
At the end <strong>of</strong> this step, we have several distribution schematic diagram solutions,<br />
which are used as a starting point for the single-line diagram. The definitive choice is<br />
confirmed at the end <strong>of</strong> the step 2.<br />
D<br />
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© Schneider Electric - all rights reserved<br />
D<br />
D - MV & LV architecture selection guide<br />
2 Simplified architecture <strong>design</strong><br />
process<br />
Step 2: choice <strong>of</strong> architecture details<br />
This involves defining the <strong>electrical</strong> <strong>installation</strong> in more detail. It is based on the<br />
results <strong>of</strong> the previous step, as well as on satisfying criteria relative to implementation<br />
and operation <strong>of</strong> the <strong>installation</strong>.<br />
The process loops back into step1 if the criteria are not satisfied. An iterative process<br />
allows several assessment criteria combinations to be analyzed.<br />
At the end <strong>of</strong> this step, we have a detailed single-line diagram.<br />
Step 3: choice <strong>of</strong> equipment<br />
The choice <strong>of</strong> equipment to be implemented is carried out in this stage, and results<br />
from the choice <strong>of</strong> architecture. The choices are made from the manufacturer<br />
catalogues, in order to satisfy certain criteria.<br />
This stage is looped back into step 2 if the characteristics are not satisfied.<br />
Assessment<br />
This assessment step allows the Engineering Office to have figures as a basis for<br />
discussions with the customer and other players.<br />
According to the result <strong>of</strong> these discussions, it may be possible to loop back into step 1.<br />
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3 Electrical <strong>installation</strong><br />
characteristics<br />
These are the main <strong>installation</strong> characteristics enabling the defining <strong>of</strong> the<br />
fundamentals and details <strong>of</strong> the <strong>electrical</strong> distribution architecture. For each <strong>of</strong> these<br />
characteristics, we supply a definition and the different categories or possible values.<br />
3.1 Activity<br />
Definition:<br />
Main economic activity carried out on the site.<br />
Indicative list <strong>of</strong> sectors considered for industrial buildings:<br />
b Manufacturing<br />
b Food & Beverage<br />
b Logistics<br />
Indicative list <strong>of</strong> sectors considered for tertiary buildings:<br />
b Offices buildings<br />
b Hypermarkets<br />
b Shopping malls<br />
3.2 Site topology<br />
Definition:<br />
Architectural characteristic <strong>of</strong> the building(s), taking account <strong>of</strong> the number <strong>of</strong><br />
buildings, number <strong>of</strong> floors, and <strong>of</strong> the surface area <strong>of</strong> each floor.<br />
Different categories:<br />
b Single storey building,<br />
b Multi-storey building,<br />
b Multi-building site,<br />
b High-rise building.<br />
3.3 Layout latitude<br />
Definition:<br />
Characteristic taking account <strong>of</strong> constraints in terms <strong>of</strong> the layout <strong>of</strong> the <strong>electrical</strong><br />
equipment in the building:<br />
b aesthetics,<br />
b accessibility,<br />
b presence <strong>of</strong> dedicated locations,<br />
b use <strong>of</strong> technical corridors (per floor),<br />
b use <strong>of</strong> technical ducts (vertical).<br />
Different categories:<br />
b Low: the position <strong>of</strong> the <strong>electrical</strong> equipment is virtually imposed<br />
b Medium: the position <strong>of</strong> the <strong>electrical</strong> equipment is partially imposed, to the<br />
detriment <strong>of</strong> the criteria to be satisfied<br />
b High: no constraints. The position <strong>of</strong> the <strong>electrical</strong> equipment can be defined to<br />
best satisfy the criteria.<br />
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3.4 Service reliability<br />
Definition:<br />
The ability <strong>of</strong> a power system to meet its supply function under stated conditions for<br />
a specified period <strong>of</strong> time.<br />
Different categories:<br />
b Minimum: this level <strong>of</strong> service reliability implies risk <strong>of</strong> interruptions related to<br />
constraints that are geographical (separate network, area distant from power<br />
production centers), technical (overhead line, poorly meshed system), or economic<br />
(insufficient maintenance, under-dimensioned generation).<br />
b Standard<br />
b Enhanced: this level <strong>of</strong> service reliability can be obtained by special measures<br />
taken to reduce the probability <strong>of</strong> interruption (underground network, strong meshing,<br />
etc.)<br />
3.5 Maintainability<br />
Definition:<br />
Features input during <strong>design</strong> to limit the impact <strong>of</strong> maintenance actions on the<br />
operation <strong>of</strong> the whole or part <strong>of</strong> the <strong>installation</strong>.<br />
Different categories:<br />
b Minimum: the <strong>installation</strong> must be stopped to carry out maintenance operations.<br />
b Standard: maintenance operations can be carried out during <strong>installation</strong><br />
operations, but with deteriorated performance. These operations must be preferably<br />
scheduled during periods <strong>of</strong> low activity. Example: several transformers with partial<br />
redundancy and load shedding.<br />
b Enhanced: special measures are taken to allow maintenance operations without<br />
disturbing the <strong>installation</strong> operations. Example: double-ended configuration.<br />
3.6 Installation flexibility<br />
Definition:<br />
Possibility <strong>of</strong> easily moving electricity delivery points within the <strong>installation</strong>, or to<br />
easily increase the power supplied at certain points. Flexibility is a criterion which<br />
also appears due to the uncertainty <strong>of</strong> the building during the pre-project summary<br />
stage.<br />
Different categories:<br />
b No flexibility: the position <strong>of</strong> loads is fixed throughout the lifecycle, due to the high<br />
constraints related to the building construction or the high weight <strong>of</strong> the supplied<br />
process. E.g.: smelting works.<br />
b Flexibility <strong>of</strong> <strong>design</strong>: the number <strong>of</strong> delivery points, the power <strong>of</strong> loads or their<br />
location are not precisely known.<br />
b Implementation flexibility: the loads can be installed after the <strong>installation</strong> is<br />
commissioned.<br />
b Operating flexibility: the position <strong>of</strong> loads will fluctuate, according to process reorganization.<br />
Examples:<br />
v industrial building: extension, splitting and changing usage<br />
v <strong>of</strong>fice building: splitting<br />
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* indicative value, supplied by standard EN50160:<br />
“Characteristics <strong>of</strong> the voltage supplied by public distribution<br />
networks”.<br />
3 Electrical <strong>installation</strong><br />
characteristics<br />
3. Power demand<br />
Definition:<br />
The sum <strong>of</strong> the apparent load power (in kVA), to which is applied a usage coefficient.<br />
This represents the maximum power which can be consumed at a given time for the<br />
<strong>installation</strong>, with the possibility <strong>of</strong> limited overloads that are <strong>of</strong> short duration.<br />
Significant power ranges correspond to the transformer power limits most commonly<br />
used:<br />
b < 630kVA<br />
b from 630 to 1250kVA<br />
b from 1250 to 2500kVA<br />
b > 2500kVA<br />
3. Load distribution<br />
Definition:<br />
A characteristic related to the uniformity <strong>of</strong> load distribution (in kVA / m²) over an area<br />
or throughout the building.<br />
Different categories:<br />
b Uniform distribution: the loads are generally <strong>of</strong> an average or low unit power and<br />
spread throughout the surface area or over a large area <strong>of</strong> the building (uniform<br />
density).<br />
E.g.: lighting, individual workstations<br />
b intermediate distribution: the loads are generally <strong>of</strong> medium power, placed in<br />
groups over the whole building surface area<br />
E.g.: machines for assembly, conveying, workstations, modular logistics “sites”<br />
b localized loads: the loads are generally high power and localized in several areas<br />
<strong>of</strong> the building (non-uniform density).<br />
E.g.: HVAC<br />
3. Power Interruption Sensitivity<br />
Definition:<br />
The aptitude <strong>of</strong> a circuit to accept a power interruption.<br />
Different categories:<br />
b “Sheddable” circuit: possible to shut down at any time for an indefinite duration<br />
b Long interruption acceptable: interruption time > 3 minutes *<br />
b Short interruption acceptable: interruption time < 3 minutes *<br />
b No interruption acceptable.<br />
We can distinguish various levels <strong>of</strong> severity <strong>of</strong> an <strong>electrical</strong> power interruption,<br />
according to the possible consequences:<br />
b No notable consequence,<br />
b Loss <strong>of</strong> production,<br />
b Deterioration <strong>of</strong> the production facilities or loss <strong>of</strong> sensitive data,<br />
b Causing mortal danger.<br />
This is expressed in terms <strong>of</strong> the criticality <strong>of</strong> supplying <strong>of</strong> loads or circuits.<br />
b Non-critical:<br />
The load or the circuit can be “shed” at any time. E.g.: sanitary water heating circuit.<br />
b Low criticality:<br />
A power interruption causes temporary discomfort for the occupants <strong>of</strong> a building,<br />
without any financial consequences. Prolonging <strong>of</strong> the interruption beyond the critical<br />
time can cause a loss <strong>of</strong> production or lower productivity. E.g.: heating, ventilation<br />
and air conditioning circuits (HVAC).<br />
b Medium criticality<br />
A power interruption causes a short break in process or service. Prolonging <strong>of</strong><br />
the interruption beyond a critical time can cause a deterioration <strong>of</strong> the production<br />
facilities or a cost <strong>of</strong> starting for starting back up.<br />
E.g.: refrigerated units, lifts.<br />
b High criticality<br />
Any power interruption causes mortal danger or unacceptable financial losses.<br />
E.g.: operating theatre, IT department, security department.<br />
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characteristics<br />
3.10 Disturbance sensitivity<br />
Definition<br />
The ability <strong>of</strong> a circuit to work correctly in presence <strong>of</strong> an <strong>electrical</strong> power<br />
disturbance.<br />
A disturbance can lead to varying degrees <strong>of</strong> malfunctioning. E.g.: stopping working,<br />
incorrect working, accelerated ageing, increase <strong>of</strong> losses, etc<br />
Types <strong>of</strong> disturbances with an impact on circuit operations:<br />
b brown-outs,<br />
b overvoltages<br />
b voltage distortion,<br />
b voltage fluctuation,<br />
b voltage imbalance.<br />
Different categories:<br />
b low sensitivity: disturbances in supply voltages have very little effect on operations.<br />
E.g.: heating device.<br />
b medium sensitivity: voltage disturbances cause a notable deterioration in<br />
operations.<br />
E.g.: motors, lighting.<br />
b high sensitivity: voltage disturbances can cause operation stoppages or even the<br />
deterioration <strong>of</strong> the supplied equipment.<br />
E.g.: IT equipment.<br />
The sensitivity <strong>of</strong> circuits to disturbances determines the <strong>design</strong> <strong>of</strong> shared or<br />
dedicated power circuits. Indeed it is better to separate “sensitive” loads from<br />
“disturbing” loads. E.g.: separating lighting circuits from motor supply circuits.<br />
This choice also depends on operating features. E.g.: separate power supply <strong>of</strong><br />
lighting circuits to enable measurement <strong>of</strong> power consumption.<br />
3.11 Disturbance capability <strong>of</strong> circuits<br />
Definition<br />
The ability <strong>of</strong> a circuit to disturb the operation <strong>of</strong> surrounding circuits due to<br />
phenomena such as: harmonics, in-rush current, imbalance, High Frequency<br />
currents, electromagnetic radiation, etc.<br />
Different categories<br />
b Non disturbing: no specific precaution to take<br />
b moderate or occasional disturbance: separate power supply may be necessary in<br />
the presence <strong>of</strong> medium or high sensitivity circuits. E.g.: lighting circuit generating<br />
harmonic currents.<br />
b Very disturbing: a dedicated power circuit or ways <strong>of</strong> attenuating disturbances are<br />
essential for the correct functioning <strong>of</strong> the <strong>installation</strong>. E.g.: <strong>electrical</strong> motor with a<br />
strong start-up current, welding equipment with fluctuating current.<br />
3.12 Other considerations or constraints<br />
b Environment<br />
E.g.: lightning classification, sun exposure<br />
b Specific <strong>rules</strong><br />
E.g.: hospitals, high rise buildings, etc.<br />
b Rule <strong>of</strong> the Energy Distributor<br />
Example: limits <strong>of</strong> connection power for LV, access to MV substation, etc<br />
b Attachment loads<br />
Loads attached to 2 independent circuits for reasons <strong>of</strong> redundancy.<br />
b Designer experience<br />
Consistency with previous <strong>design</strong>s or partial usage <strong>of</strong> previous <strong>design</strong>s,<br />
standardization <strong>of</strong> sub-assemblies, existence <strong>of</strong> an installed equipment base.<br />
b Load power supply constraints<br />
Voltage level (230V, 400V, 690V), voltage system (single-phase, three-phase with or<br />
without neutral, etc)<br />
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Fig. D4 : Different index values<br />
4 Technological characteristics<br />
The technological solutions considered concern the various types <strong>of</strong> MV and LV<br />
equipment, as well as Busbar Trunking Systems .<br />
The choice <strong>of</strong> technological solutions is made following the choice <strong>of</strong> single-line<br />
diagram and according to characteristics given below.<br />
4.1 Environment, atmosphere<br />
A notion taking account <strong>of</strong> all <strong>of</strong> the environmental constraints (average ambient<br />
temperature, altitude, humidity, corrosion, dust, impact, etc.) and bringing together<br />
protection indexes IP and IK.<br />
Different categories:<br />
b Standard: no particular environmental constraints<br />
b Enhanced: severe environment, several environmental parameters generate<br />
important constraints for the installed equipment<br />
b Specific: atypical environment, requiring special enhancements<br />
4.2 Service Index<br />
The service index (IS) is a value that allows us to characterize an LV switchboard<br />
according to user requirements in terms <strong>of</strong> operation, maintenance, and scalability.<br />
The different index values are indicated in the following table (Fig D4):<br />
Operation Maintenance Upgrade<br />
Level 1 IS = 1 • •<br />
Operation may lead to complete<br />
stoppage <strong>of</strong> the switchboard<br />
Level 2 IS = 2 • •<br />
Operation may lead to stoppage <strong>of</strong><br />
only the functional unit<br />
Level 3 IS = 3 • •<br />
Operation may lead to stoppage <strong>of</strong><br />
the power <strong>of</strong> the functional unit only<br />
IS = • 1 •<br />
Operation may lead to complete<br />
stoppage <strong>of</strong> the switchboard<br />
IS = • 2 •<br />
Operation may lead to stoppage <strong>of</strong><br />
only the functional unit, with work on<br />
connections<br />
IS = • 3 •<br />
Operation may lead to stoppage <strong>of</strong><br />
only the functional unit, without work<br />
on connections<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
IS = • • 1<br />
Operation may lead to complete<br />
stoppage <strong>of</strong> the switchboard<br />
IS = • • 2<br />
Operation may lead to stoppage<br />
<strong>of</strong> only the functional unit, with<br />
functional units provided for back-up<br />
IS = • • 3<br />
Operation may lead to stoppage <strong>of</strong><br />
only the functional unit, with total<br />
freedom in terms <strong>of</strong> upgrade<br />
b Examples <strong>of</strong> an operation event: turning <strong>of</strong>f a circuit-breaker, switching operation to<br />
energize/de-energize a machine<br />
b Example <strong>of</strong> a maintenance operation: tightening connections<br />
b Example <strong>of</strong> an upgrade operation: connecting an additional feeder<br />
There are a limited number <strong>of</strong> relevant service indices (see Fig. D5)<br />
IS Operation Maintenance Upgrade<br />
1 1 1 Switching <strong>of</strong>f the whole switchboard Working time > 1h, with total nonavailability<br />
Extension not planned<br />
2 1 1<br />
2 2 3 Working time between 1/4h and 1h,<br />
with work on connections<br />
2 3 2<br />
Individually switching <strong>of</strong>f the functional<br />
unit and re-commissioning < 1h<br />
Working time between 1/4h and 1h,<br />
without work on connections<br />
Possible adding <strong>of</strong> functional units<br />
without stopping the switchboard<br />
Possible adding <strong>of</strong> functional units with<br />
stopping the switchboard<br />
2 3 3 Possible adding <strong>of</strong> functional units<br />
without stopping the switchboard<br />
3 3 2<br />
Possible adding <strong>of</strong> functional units with<br />
stopping the switchboard<br />
3 3 3<br />
Individually switching <strong>of</strong>f the functional<br />
unit and re-commissioning < 1/4h<br />
Possible adding <strong>of</strong> functional units<br />
without stopping the switchboard<br />
Fig. D5 : Relevant service indices (IS)<br />
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Service rating Protection index<br />
IP<br />
4 Technological characteristics<br />
The types <strong>of</strong> <strong>electrical</strong> connections <strong>of</strong> functional units can be denoted by a threeletter<br />
code:<br />
b The first letter denotes the type <strong>of</strong> <strong>electrical</strong> connection <strong>of</strong> the main incoming<br />
circuit,<br />
b The second letter denotes the type <strong>of</strong> <strong>electrical</strong> connection <strong>of</strong> the main outgoing<br />
circuit,<br />
b The third letter denotes the type <strong>of</strong> <strong>electrical</strong> connection <strong>of</strong> the auxiliary circuits.<br />
The following letters are used:<br />
b F for fixed connections,<br />
b D for disconnectable connections,<br />
b W for withdrawable connections.<br />
Service ratings are related to other mechanical parameters, such as the Protection<br />
Index (IP), form <strong>of</strong> internal separations, the type <strong>of</strong> connection <strong>of</strong> functional units or<br />
switchgear (Fig. D6):<br />
Form Functional Unit<br />
Withdrawability<br />
1 1 1 2 X X 1 F F F<br />
2 1 1 2 X B 1 F F F<br />
2 2 3 2 X B 3b W F D<br />
2 3 2 2 X B 3b W F W<br />
2 3 3 2 X B 3b W W W<br />
3 3 2 2 X B 3b W W W<br />
3 3 3 2 X B 3b W W W<br />
Fig. D6 : Correspondence between service index and other mechanical parameters<br />
Technological examples are given in chapter E2.<br />
b Definition <strong>of</strong> the protection index: see IEC 60529: “Degree <strong>of</strong> protection given by<br />
enclosures (IP code)”,<br />
b Definitions <strong>of</strong> the form and withdrawability: see IEC 60439-1: “Low-voltage<br />
switchgear and controlgear assemblies; part 1: type-tested and partially type-tested<br />
assemblies”.<br />
4.3 Other considerations<br />
Other considerations have an impact on the choice <strong>of</strong> technological solutions:<br />
b Designer experience,<br />
b Consistency with past <strong>design</strong>s or the partial use <strong>of</strong> past <strong>design</strong>s,<br />
b Standardization <strong>of</strong> sub-assemblies,<br />
b The existence <strong>of</strong> an installed equipment base,<br />
b Utilities requirements,<br />
b Technical criteria: target power factor, backed-up load power, presence <strong>of</strong> harmonic<br />
generators…<br />
These considerations should be taken into account during the detailed <strong>electrical</strong><br />
definition phase following the draft <strong>design</strong> stage.<br />
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5 Architecture assessment criteria<br />
Certain decisive criteria are assessed at the end <strong>of</strong> the 3 stages in defining<br />
architecture, in order to validate the architecture choice. These criteria are listed<br />
below with the different allocated levels <strong>of</strong> priority.<br />
5.1 On-site work time<br />
Time for implementing the <strong>electrical</strong> equipment on the site.<br />
Different levels <strong>of</strong> priority:<br />
b Secondary: the on-site work time can be extended, if this gives a reduction in<br />
overall <strong>installation</strong> costs,<br />
b Special: the on-site work time must be minimized, without generating any<br />
significant excess cost,<br />
b Critical: the on-site work time must be reduced as far as possible, imperatively,<br />
even if this generates a higher total <strong>installation</strong> cost,<br />
5.2 Environmental impact<br />
Taking into consideration environmental constraints in the <strong>installation</strong> <strong>design</strong>. This<br />
takes account <strong>of</strong>: consumption <strong>of</strong> natural resources, Joule losses (related to CO 2<br />
emission), “recyclability” potential, throughout the <strong>installation</strong>’s lifecycle.<br />
Different levels <strong>of</strong> priority:<br />
b Non significant: environmental constraints are not given any special consideration,<br />
b Minimal: the <strong>installation</strong> is <strong>design</strong>ed with minimum regulatory requirements,<br />
b Proactive: the <strong>installation</strong> is <strong>design</strong>ed with a specific concern for protecting<br />
the environment. Excess cost is allowed in this situation. E.g.: using low-loss<br />
transformers.<br />
The environmental impact <strong>of</strong> an <strong>installation</strong> will be determined according to the<br />
method carrying out an <strong>installation</strong> lifecycle analysis, in which we distinguish<br />
between the following 3 phases:<br />
b manufacture,<br />
b operation,<br />
b end <strong>of</strong> life (dismantling, recycling).<br />
In terms <strong>of</strong> environmental impact, 3 indicators (at least) can be taken into account<br />
and influenced by the <strong>design</strong> <strong>of</strong> an <strong>electrical</strong> <strong>installation</strong>. Although each lifecycle<br />
phase contributes to the three indicators, each <strong>of</strong> these indicators is mainly related to<br />
one phase in particular:<br />
b consumption <strong>of</strong> natural resources mainly has an impact on the manufacturing<br />
phase,<br />
b consumption <strong>of</strong> energy has an impact on the operation phase,<br />
b “recycleability” potential has an impact on the end <strong>of</strong> life.<br />
The following table details the contributing factors to the 3 environmental indicators<br />
(Fig D7).<br />
Indicators Contributors<br />
Natural resources consumption Mass and type <strong>of</strong> materials used<br />
Power consumption Joule losses at full load and no load<br />
«Recyclability» potential Mass and type <strong>of</strong> material used<br />
Fig D7 : Contributing factors to the 3 environmental indicators<br />
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5.3 Preventive maintenance level<br />
Definition:<br />
Number <strong>of</strong> hours and sophistication <strong>of</strong> maintenance carried out during operations in<br />
conformity with manufacturer recommendations to ensure dependable operation <strong>of</strong><br />
the <strong>installation</strong> and the maintaining <strong>of</strong> performance levels (avoiding failure: tripping,<br />
down time, etc).<br />
Different categories:<br />
b Standard: according to manufacturer recommendations.<br />
b Enhanced: according to manufacturer recommendations, with a severe<br />
environment,<br />
b Specific: specific maintenance plan, meeting high requirements for continuity <strong>of</strong><br />
service, and requiring a high level <strong>of</strong> maintenance staff competency.<br />
5.4 Availability <strong>of</strong> <strong>electrical</strong> power supply<br />
Definition:<br />
This is the probability that an <strong>electrical</strong> <strong>installation</strong> be capable <strong>of</strong> supplying quality<br />
power in conformity with the specifications <strong>of</strong> the equipment it is supplying. This is<br />
expressed by an availability level:<br />
Availability (%) = (1 - MTTR/ MTBF) x 100<br />
MTTR (Mean Time To Repair): the average time to make the <strong>electrical</strong> system once<br />
again operational following a failure (this includes detection <strong>of</strong> the reason for failure,<br />
its repair and re-commissioning),<br />
MTBF (Mean Time Between Failure): measurement <strong>of</strong> the average time for which<br />
the <strong>electrical</strong> system is operational and therefore enables correct operation <strong>of</strong> the<br />
application.<br />
The different availability categories can only be defined for a given type <strong>of</strong><br />
<strong>installation</strong>. E.g.: hospitals, data centers.<br />
Example <strong>of</strong> classification used in data centers:<br />
Tier 1: the power supply and air conditioning are provided by one single channel,<br />
without redundancy, which allows availability <strong>of</strong> 99.671%,<br />
Tier 2: the power supply and air conditioning are provided by one single channel,<br />
with redundancy, which allows availability <strong>of</strong> 99.741%,<br />
Tier 3: the power supply and air conditioning are provided by several channels, with<br />
one single redundant channel, which allows availability <strong>of</strong> 99.982%,<br />
Tier 4: the power supply and air conditioning are provided by several channels, with<br />
redundancy, which allows availability <strong>of</strong> 99.995%.<br />
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6 Choice <strong>of</strong> architecture<br />
fundamentals<br />
The single-line diagram can be broken down into different key parts, which are<br />
determined throughout a process in 2 successive stages. During the first stage we<br />
make the following choices:<br />
b connection to the utilities network,<br />
b configuration <strong>of</strong> MV circuits,<br />
b number <strong>of</strong> power transformers,<br />
b number and distribution <strong>of</strong> transformation substations,<br />
b MV back-up generator<br />
6.1 Connection to the upstream network<br />
The main configurations for possible connection are as follows (see Fig. D8 for MV<br />
service):<br />
b LV service,<br />
b MV single-line service,<br />
b MV ring-main service,<br />
b MV duplicate supply service,<br />
b MV duplicate supply service with double busbar.<br />
Metering, protection, disconnection devices, located in the delivery substations are<br />
not represented on the following diagrams. They are <strong>of</strong>ten specific to each utilities<br />
company and do not have an influence on the choice <strong>of</strong> <strong>installation</strong> architecture.<br />
For each connection, one single transformer is shown for simplification purposes, but<br />
in the practice, several transformers can be connected.<br />
(MLVS: Main Low Voltage Switchboard)<br />
a) Single-line: b) Ring-main:<br />
MV<br />
LV<br />
MLVS<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
MLVS<br />
c) Duplicate supply: d) Double busbar with duplicate supply:<br />
MV<br />
LV<br />
MLVS<br />
Fig. D8 : MV connection to the utilities network<br />
MV<br />
LV<br />
MV<br />
LV<br />
MLVS1<br />
MV<br />
LV<br />
MLVS2<br />
D15<br />
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D16<br />
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D - MV & LV architecture selection guide<br />
Characteristic to<br />
consider<br />
Configuration<br />
LV MV<br />
For the different possible configurations, the most probable and usual set <strong>of</strong><br />
characteristics is given in the following table:<br />
Simple-line Ring-main Duplicate supply Duplicate supply<br />
with double<br />
busbars<br />
Activity Any Any Any Hi-tech, sensitive<br />
<strong>of</strong>fice, health-care<br />
Site topology Single building Single building Single building Single building Several buildings<br />
Service reliability Minimal Minimal Standard Enhanced Enhanced<br />
Power demand < 630kVA ≤ 1250kVA ≤ 2500kVA > 2500kVA > 2500kVA<br />
Other connection<br />
constraints<br />
Any Isolated site Low density urban<br />
area<br />
a) Single feeder: b) Open ring, 1 MV substation:<br />
MV MV MV MV MV<br />
LV<br />
MLVS 1<br />
LV<br />
MLVS n<br />
Fig. D9 : MV circuit configuration<br />
LV<br />
MLVS 1<br />
6 Choice <strong>of</strong> architecture<br />
fundamentals<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
High density<br />
urban area<br />
6.2 MV circuit configuration<br />
Any<br />
Urban area with<br />
utility constraint<br />
The main possible connection configurations are as follows (Fig. D9):<br />
b single feeder, one or several transformers<br />
b open ring, one MV incomer<br />
b open ring, 2 MV incomers<br />
The basic configuration is a radial single-feeder architecture, with one single<br />
transformer.<br />
In the case <strong>of</strong> using several transformers, no ring is realised unless all <strong>of</strong> the<br />
transformers are located in a same substation.<br />
Closed-ring configuration is not taken into account.<br />
LV<br />
MLVS 2<br />
LV<br />
MLVS n<br />
c) Open ring, 2 MV substations:<br />
MV<br />
LV<br />
MLVS 1<br />
MV<br />
LV<br />
MLVS 2<br />
MV<br />
LV<br />
MLVS n
D - MV & LV architecture selection guide<br />
6 Choice <strong>of</strong> architecture<br />
fundamentals<br />
For the different possible configurations, the most probable and usual set <strong>of</strong><br />
characteristics is given in the table on Fig D10.<br />
Characteristic to<br />
consider<br />
Site topology Any<br />
< 25000m²<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
MV circuit configuration<br />
Single feeder Open ring<br />
1 MV substation<br />
Building with one<br />
level or several<br />
buildings<br />
≤ 25000m²<br />
Open ring<br />
2 MV substations<br />
Several buildings<br />
≥ 25000m²<br />
Maintainability Minimal or standard Enhanced Enhanced<br />
Power demand Any > 1250kVA > 2500kVA<br />
Disturbance sensitivity Long interruption<br />
acceptable<br />
Fig. D10 : Typical values <strong>of</strong> the <strong>installation</strong> characteristics<br />
Short interruption<br />
acceptable<br />
Short interruption<br />
acceptable<br />
Another exceptional configuration: power supply by 2 MV substations and connection<br />
<strong>of</strong> the transformers to each <strong>of</strong> these 2 substations (MV “double ended” connection).<br />
6.3 Number and distribution <strong>of</strong> MV/LV<br />
transformation substations<br />
Main characteristics to consider to determine the transformation substations:<br />
b Surface area <strong>of</strong> building or site<br />
b Power demand, (to be compared with standardized transformer power),<br />
b Load distribution<br />
The preferred basic configuration comprises one single substation. Certain factors<br />
contribute to increasing the number <strong>of</strong> substations (> 1):<br />
b A large surface area (> 25000m²),<br />
b The site configuration: several buildings,<br />
b Total power > 2500kVA,<br />
b Sensitivity to interruption: need for redundancy in the case <strong>of</strong> a fire.<br />
Characteristic to<br />
consider<br />
Configuration<br />
1 substation with<br />
N transformers<br />
N substations<br />
N transformers<br />
(identical substations)<br />
Building configuration < 25000m² ≥ 25000m²<br />
1 building with several<br />
floors<br />
N substations<br />
M transformers<br />
(different powers)<br />
≥ 25000m²<br />
several buildings<br />
Power demand < 2500kVA ≥ 2500kVA ≥ 2500kVA<br />
Load distribution Localized loads Uniform distribution Medium density<br />
Fig. D11 : Typical characteristics <strong>of</strong> the different configurations<br />
D17<br />
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D18<br />
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D - MV & LV architecture selection guide<br />
6 Choice <strong>of</strong> architecture<br />
fundamentals<br />
6.4 Number <strong>of</strong> MV/LV transformers<br />
Main characteristics to consider to determine the number <strong>of</strong> transformers:<br />
b Surface <strong>of</strong> building or site<br />
b Total power <strong>of</strong> the installed loads<br />
b Sensitivity <strong>of</strong> circuits to power interruptions<br />
b Sensitivity <strong>of</strong> circuits to disturbances<br />
b Installation scalability<br />
The basic preferred configuration comprises a single transformer supplying the total<br />
power <strong>of</strong> the installed loads. Certain factors contribute to increasing the number <strong>of</strong><br />
transformers (> 1), preferably <strong>of</strong> equal power:<br />
b A high total installed power (> 1250kVA): practical limit <strong>of</strong> unit power<br />
(standardization, ease <strong>of</strong> replacement, space requirement, etc),<br />
b A large surface area (> 5000m²): the setting up <strong>of</strong> several transformers as close as<br />
possible to the distributed loads allows the length <strong>of</strong> LV trunking to be reduced<br />
b A need for partial redundancy (down-graded operation possible in the case <strong>of</strong> a<br />
transformer failure) or total redundancy (normal operation ensured in the case a<br />
transformer failure)<br />
b Separating <strong>of</strong> sensitive and disturbing loads (e.g.: IT, motors)<br />
6.5 MV back-up generator<br />
Main characteristics to consider for the implementation <strong>of</strong> an MV back-up generator:<br />
b Site activity<br />
b Total power <strong>of</strong> the installed loads<br />
b Sensitivity <strong>of</strong> circuits to power interruptions<br />
b Availability <strong>of</strong> the public distribution network<br />
The preferred basic configuration does not include an MV generator. Certain factors<br />
contribute to installing an MV generator:<br />
b Site activity: process with co-generation, optimizing the energy bill,<br />
b Low availability <strong>of</strong> the public distribution network.<br />
Installation <strong>of</strong> a back-up generator can also be carried out at LV level.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
D - MV & LV architecture selection guide<br />
Office<br />
Finishing<br />
7 Choice <strong>of</strong> architecture details<br />
This is the second stage in <strong>design</strong>ing <strong>of</strong> the <strong>electrical</strong> <strong>installation</strong>. During this stage<br />
we carry out the following choices are carried out:<br />
b Layout,<br />
b Centralized or decentralized distribution,<br />
b Presence <strong>of</strong> back-up generators,<br />
b Presence <strong>of</strong> uninterruptible power supplies,<br />
b Configuration <strong>of</strong> LV circuits,<br />
b Architecture combinations.<br />
7.1 Layout<br />
Position <strong>of</strong> the main MV and LV equipment on the site or in the building.<br />
This layout choice is applied to the results <strong>of</strong> stage 1.<br />
Selection guide:<br />
b Place power sources as close as possible to the barycenter <strong>of</strong> power consumers,<br />
b Reduce atmospheric constraints: building dedicated premises if the layout in the<br />
workshop is too restrictive (temperature, vibrations, dust, etc.),<br />
b Placing heavy equipment (transformers, generators, etc) close to walls or main<br />
exists for ease <strong>of</strong> maintenance,<br />
A layout example is given in the following diagram (Fig. D12):<br />
Global current<br />
consumer<br />
Barycenter<br />
Painting<br />
Fig. D12 : The position <strong>of</strong> the global current consumer barycenter guides the positioning <strong>of</strong> power sources<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Panel<br />
shop<br />
D19<br />
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D20<br />
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D - MV & LV architecture selection guide<br />
7 Choice <strong>of</strong> architecture details<br />
7.2 Centralized or distributed layout<br />
In centralized layout, current consumers are connected to the power sources<br />
by a star-connection. Cables are suitable for centralized layout, with point to point<br />
links between the MLVS and current consumers or sub-distribution boards (radial<br />
distribution, star- distribution) (Fig. D13):<br />
Fig. D13: Example <strong>of</strong> centralized layout with point to point links<br />
In decentralized layout, current consumers are connected to sources via a busway.<br />
Busbar trunking systems are well suited to decentralized layout, to supply many<br />
loads that are spread out, making it easy to change, move or add connections<br />
(Fig D14):<br />
Fig. D14 : Example <strong>of</strong> decentralized layout, with busbar trunking links<br />
Factors in favour <strong>of</strong> centralized layout (see summary table in Fig. D15):<br />
b Installation flexibility: no,<br />
b Load distribution: localized loads (high unit power loads).<br />
Factors in favor <strong>of</strong> decentralized layout:<br />
b Installation flexibility: "Implementation" flexibility (moving <strong>of</strong> workstations, etc…),<br />
b Load distribution: uniform distribution <strong>of</strong> low unit power loads<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
D - MV & LV architecture selection guide<br />
7 Choice <strong>of</strong> architecture details<br />
Load distribution<br />
Flexibility Localized loads Intermediate<br />
distribution<br />
No flexibility<br />
Design flexibility<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Uniform distributed<br />
Centralized Decentralized<br />
Implementation<br />
flexibility Centralized Decentralized<br />
Operation flexibility<br />
Fig. D15 : Recommendations for centralized or decentralized layout<br />
Power supply by cables gives greater independence <strong>of</strong> circuits (lighting, power<br />
sockets, HVAC, motors, auxiliaries, security, etc), reducing the consequences <strong>of</strong> a<br />
fault from the point <strong>of</strong> view <strong>of</strong> power availability.<br />
The use <strong>of</strong> busbar trunking systems allows load power circuits to be combined and<br />
saves on conductors by taking advantage <strong>of</strong> a clustering coefficient. The choice<br />
between cable and busbar trunking, according to the clustering coefficient, allows us<br />
to find an economic optimum between investment costs, implementation costs and<br />
operating costs.<br />
These two distribution modes are <strong>of</strong>ten combined.<br />
Presence <strong>of</strong> back-up generators (Fig. D16)<br />
Here we only consider LV back-up generators.<br />
The <strong>electrical</strong> power supply supplied by a back-up generator is produced by an<br />
alternator, driven by a thermal engine.<br />
No power can be produced until the generator has reached its rated speed. This type<br />
<strong>of</strong> device is therefore not suitable for an uninterrupted power supply.<br />
According to the generator’s capacity to supply power to all or only part <strong>of</strong> the<br />
<strong>installation</strong>, there is either total or partial redundancy.<br />
A back-up generator functions generally disconnected from the network. A source<br />
switching system is therefore necessary.<br />
The generator can function permanently or intermittently. Its back-up time depends<br />
on the quantity <strong>of</strong> available fuel.<br />
Fig. D16 : Connection <strong>of</strong> a back-up generator<br />
LV switchboard<br />
The main characteristics to consider for implementing LV back-up generator:<br />
b Sensitivity <strong>of</strong> loads to power interruption,<br />
b Availability <strong>of</strong> the public distribution network,<br />
b Other constraints (e.g.: generators compulsory in hospitals or high-vise buildings)<br />
The presence <strong>of</strong> generators can be decided to reduce the energy bill or due to the<br />
opportunity for co-generation. These two aspects are not taken into account in this<br />
guide.<br />
The presence <strong>of</strong> a back-up generator is essential if the loads cannot be shed for<br />
an indefinite duration (long interruption only acceptable) or if the utility network<br />
availability is low.<br />
Determining the number <strong>of</strong> back-up generator units is in line with the same criteria<br />
as determining the number <strong>of</strong> transformers, as well as taking account <strong>of</strong> economic<br />
and availability considerations (redundancy, start-up reliability, maintenance facility).<br />
G<br />
D21<br />
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D22<br />
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D - MV & LV architecture selection guide<br />
MLVS<br />
Fig. D18 : Radial single feeder configuration<br />
MLVS<br />
Fig. D19 : Two-pole configuration<br />
MLVS<br />
NO<br />
Fig. D20 : Two-pole configuration with two ½ MLVS and NO link<br />
7 Choice <strong>of</strong> architecture details<br />
7.3 Presence <strong>of</strong> an Uninterruptible Power Supply (UPS)<br />
The <strong>electrical</strong> power from a UPS is supplied from a storage unit: batteries or inertia<br />
wheel. This system allows us to avoid any power failure. The back-up time <strong>of</strong> the<br />
system is limited: from several minutes to several hours.<br />
The simultaneous presence <strong>of</strong> a back-up generator and a UPS unit is used for<br />
permanently supply loads for which no failure is acceptable (Fig. D17). The back-up<br />
time <strong>of</strong> the battery or the inertia wheel must be compatible with the maximum time<br />
for the generator to start up and be brought on-line.<br />
A UPS unit is also used for supply power to loads that are sensitive to disturbances<br />
(generating a “clean” voltage that is independent <strong>of</strong> the network).<br />
Main characteristics to be considered for implementing a UPS:<br />
b Sensitivity <strong>of</strong> loads to power interruptions,<br />
b Sensitivity <strong>of</strong> loads to disturbances.<br />
The presence <strong>of</strong> a UPS unit is essential if and only if no failure is acceptable.<br />
LV Switchboard<br />
Non-critical<br />
circuit<br />
Fig. D17 : Example <strong>of</strong> connection for a UPS<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Normal<br />
ASI<br />
Critical<br />
circuit<br />
7.4 Configuration <strong>of</strong> LV circuits<br />
G<br />
By-pass<br />
Main possible configurations (see figures D18 to D25):<br />
b Radial single feeder configuration: This is the reference configuration and<br />
the most simple. A load is connected to only one single source. This configuration<br />
provides a minimum level <strong>of</strong> availability, since there is no redundancy in case <strong>of</strong><br />
power source failure.<br />
b Two-pole configuration: The power supply is provided by 2 transformers,<br />
connected to the same MV line. When the transformers are close, they are generally<br />
connected in parallel to the same MLVS.<br />
b Variant: two-pole with two ½ MLVS: In order to increase the availability in case<br />
<strong>of</strong> failure <strong>of</strong> the busbars or authorize maintenance on one <strong>of</strong> the transformers,<br />
it is possible to split the MLVS into 2 parts, with a normally open link (NO). This<br />
configuration generally requires an Automatic Transfer Switch, (ATS).<br />
b Shedable switchboard (simple disconnectable attachment): A series <strong>of</strong><br />
shedable circuits can be connected to a dedicated switchboard. The connection to<br />
the MLVS is interrupted when needed (overload, generator operation, etc)<br />
b Interconnected switchboards: If transformers are physically distant from one<br />
another, they may be connected by a busbar trunking. A critical load can be supplied<br />
by one or other <strong>of</strong> the transformers. The availability <strong>of</strong> power is therefore improved,<br />
since the load can always be supplied in the case <strong>of</strong> failure <strong>of</strong> one <strong>of</strong> the sources.<br />
The redundancy can be:<br />
v Total: each transformer being capable <strong>of</strong> supplying all <strong>of</strong> the <strong>installation</strong>,<br />
v Partial: each transformer only being able to supply part <strong>of</strong> the <strong>installation</strong>. In this<br />
case, part <strong>of</strong> the loads must be disconnected (load-shedding) in the case <strong>of</strong> one <strong>of</strong><br />
the transformers failing.
D - MV & LV architecture selection guide<br />
LV swichboard<br />
Fig. D21 : Shedable switchboard<br />
MLVS<br />
Busbar<br />
Fig. D22 : Interconnected switchboards<br />
Busbar<br />
MLVS<br />
MLVS<br />
Fig. D23 : Ring configuration<br />
Busbar<br />
Busbar<br />
MLVS<br />
MLVS<br />
MLVS<br />
MLVS<br />
Busbar<br />
7 Choice <strong>of</strong> architecture details<br />
b Ring configuration: This configuration can be considered as an extension <strong>of</strong> the<br />
configuration with interconnection between switchboards. Typically, 4 transformers<br />
connected to the same MV line, supply a ring using busbar trunking. A given load<br />
is then supplied power by several clustered transformers. This configuration is well<br />
suited to extended <strong>installation</strong>s, with a high load density (in kVA/m²). If all <strong>of</strong> the loads<br />
can be supplied by 3 transformers, there is total redundancy in the case <strong>of</strong> failure<br />
<strong>of</strong> one <strong>of</strong> the transformers. In fact, each busbar can be fed power by one or other<br />
<strong>of</strong> its ends. Otherwise, downgraded operation must be considered (with partial load<br />
shedding). This configuration requires special <strong>design</strong> <strong>of</strong> the protection plan in order<br />
to ensure discrimination in all <strong>of</strong> the fault circumstances.<br />
b Double-ended power supply: This configuration is implemented in cases where<br />
maximum availability is required. The principle involves having 2 independent power<br />
sources, e.g.:<br />
v 2 transformers supplied by different MV lines,<br />
v 1 transformer and 1 generator,<br />
v 1 transformer and 1 UPS.<br />
An automatic transfer switch (ATS) is used to avoid the sources being parallel<br />
connected. This configuration allows preventive and curative maintenance to be<br />
carried out on all <strong>of</strong> the <strong>electrical</strong> distribution system upstream without interrupting<br />
the power supply.<br />
b Configuration combinations: An <strong>installation</strong> can be made up <strong>of</strong> several subasssemblies<br />
with different configurations, according to requirements for the<br />
availability <strong>of</strong> the different types <strong>of</strong> load. E.g.: generator unit and UPS, choice by<br />
sectors (some sectors supplied by cables and others by busbar trunking).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
or G or UPS<br />
Fig. D24 : Double-ended configuration with automatic transfer switch<br />
1<br />
MLVS<br />
2 3<br />
Busbar<br />
MLVS<br />
Fig. D25 : Example <strong>of</strong> a configuration combination<br />
1: Single feeder, 2: Switchboard interconnection, 3: Double-ended<br />
G<br />
D23<br />
© Schneider Electric - all rights reserved
D24<br />
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D - MV & LV architecture selection guide<br />
Characteristic to be<br />
considered<br />
Configuration<br />
7 Choice <strong>of</strong> architecture details<br />
For the different possible configurations, the most probable and usual set <strong>of</strong><br />
characteristics is given in the following table:<br />
Radial Two-pole Sheddable load Interconnected<br />
switchboards<br />
Site topology Any Any Any 1 level<br />
5 to 25000m²<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Ring Double-ended<br />
1 level<br />
5 to 25000m²<br />
Location latitude Any Any Any Medium <strong>of</strong> high Medium or high Any<br />
Maintainability Minimal Standard Minimal Standard Standard Enhanced<br />
Power demand < 2500kVA Any Any ≥ 1250kVA > 2500kVA Any<br />
Load distribution Localized loads Localized loads Localized load Intermediate or<br />
uniforme distribution<br />
Interruptions sensitivity Long<br />
interruption<br />
acceptable<br />
Long<br />
interruption<br />
acceptable<br />
Sheddable Long<br />
interruption<br />
acceptable<br />
Any<br />
Uniform distribution Localized loads<br />
Long<br />
interruption<br />
acceptable<br />
Short or no<br />
interruption<br />
Disturbances sensitivity Low sensitivity High sensitivity Low sensitivity High sensitivity High sensitivity High sensitivity<br />
Other constraints / / / / / Double-ended<br />
loads
D - MV & LV architecture selection guide<br />
8 Choice <strong>of</strong> equipment<br />
The choice <strong>of</strong> equipment is step 3 in the <strong>design</strong> <strong>of</strong> an <strong>electrical</strong> <strong>installation</strong>. The aim<br />
<strong>of</strong> this step is to select equipment from the manufacturers’ catalogues. The choice <strong>of</strong><br />
technological solutions results from the choice <strong>of</strong> architecture.<br />
List <strong>of</strong> equipment to consider:<br />
b MV/LV substation,<br />
b MV switchboards,<br />
b Transformers,<br />
b LV switchboards,<br />
b Busbar trunking,<br />
b UPS units,<br />
b Power factor correction and filtering equipment.<br />
Criteria to consider:<br />
b Atmosphere, environment,<br />
b Service index,<br />
b Offer availability per country,<br />
b Utilities requirements,<br />
b Previous architecture choices.<br />
The choice <strong>of</strong> equipment is basically linked to the <strong>of</strong>fer availability in the country. This<br />
criterion takes into account the availability <strong>of</strong> certain ranges <strong>of</strong> equipment or local<br />
technical support.<br />
The detailed selection <strong>of</strong> equipment is out <strong>of</strong> the scope <strong>of</strong> this document.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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D26<br />
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D - MV & LV architecture selection guide<br />
9 Recommendations for<br />
architecture optimization<br />
These recommendations are intended to guide the <strong>design</strong>er towards architecture<br />
upgrades which allow him to improve assessment criteria.<br />
9.1 On-site work<br />
To be compatible with the “special” or “critical” work-site time, it is recommended to<br />
limit uncertainties by applying the following recommendations:<br />
b Use <strong>of</strong> proven solutions and equipment that has been validated and tested by<br />
manufacturers (“functional” switchboard or “manufacturer” switchboard according to<br />
the application criticality),<br />
b Prefer the implementation <strong>of</strong> equipment for which there is a reliable distribution<br />
network and for which it is possible to have local support (supplier well established),<br />
b Prefer the use <strong>of</strong> factory-built equipment (MV/LV substation, busbar trunking),<br />
allowing the volume <strong>of</strong> operations on site to be limited,<br />
b Limit the variety <strong>of</strong> equipment implemented (e.g. the power <strong>of</strong> transformers),<br />
b Avoid mixing equipment from different manufacturers.<br />
9.2 Environmental impact<br />
The optimization <strong>of</strong> the environmental assessment <strong>of</strong> an <strong>installation</strong> will involve<br />
reducing:<br />
b Power losses at full load and no load during <strong>installation</strong> operation,<br />
b Overall, the mass <strong>of</strong> materials used to produce the <strong>installation</strong>.<br />
Taken separately and when looking at only one piece <strong>of</strong> equipment, these 2<br />
objectives may seem contradictory. However, when applied to whole <strong>installation</strong>, it<br />
is possible to <strong>design</strong> the architecture to contribute to both objectives. The optimal<br />
<strong>installation</strong> will therefore not be the sum <strong>of</strong> the optimal equipment taken separately,<br />
but the result <strong>of</strong> an optimization <strong>of</strong> the overall <strong>installation</strong>.<br />
Figure D26 gives an example <strong>of</strong> the contribution per equipment category to the<br />
weight and energy dissipation for a 3500 kVA <strong>installation</strong> spread over 10000m².<br />
LV switchboard<br />
and switchgear<br />
5 %<br />
LV cables<br />
and trunking<br />
75 %<br />
Transformers<br />
20 %<br />
10 %<br />
LV cables<br />
and trunking<br />
46 %<br />
Transformers<br />
44 %<br />
Total loss for equipment considered: 414 MWh Total mass <strong>of</strong> equipment considered: 18,900 kg<br />
Fig. D26 : Example <strong>of</strong> the spread <strong>of</strong> losses and the weight <strong>of</strong> material for each equipment category<br />
LV switchboard<br />
and switchgear<br />
<strong>General</strong>ly speaking, LV cables and trunking as well as the MV/LV transformers are<br />
the main contributors to operating losses and the weight <strong>of</strong> equipment used.<br />
Environmental optimization <strong>of</strong> the <strong>installation</strong> by the architecture will therefore<br />
involve:<br />
b reducing the length <strong>of</strong> LV circuits in the <strong>installation</strong>,<br />
b clustering LV circuits wherever possible to take advantage <strong>of</strong> the factor <strong>of</strong><br />
simultaneity ks (see chapter A: <strong>General</strong> <strong>rules</strong> <strong>of</strong> <strong>electrical</strong> <strong>installation</strong> <strong>design</strong>, <strong>Chapter</strong><br />
– Power loading <strong>of</strong> an <strong>installation</strong>, 4.3 “Estimation <strong>of</strong> actual maximum kVA demand”)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
D - MV & LV architecture selection guide<br />
Objectives Resources<br />
Reducing the length <strong>of</strong> LV<br />
circuits<br />
Solution Barycenter position<br />
N°1<br />
N°2<br />
Fig. D28 : Example <strong>of</strong> barycenter positioning<br />
MLVS area<br />
MLVS area<br />
Workshop 1<br />
Workshop 1<br />
Barycenter<br />
Barycenter<br />
line 1<br />
Workshop 1<br />
9 Recommendations for<br />
architecture optimization<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Placing MV/LV substations as close as possible to the barycenter<br />
<strong>of</strong> all <strong>of</strong> the LV loads to be supplied<br />
Clustering LV circuits When the simultaneity factor <strong>of</strong> a group <strong>of</strong> loads to be supplied<br />
is less than 0.7, the clustering <strong>of</strong> circuits allows us to limit the<br />
volume <strong>of</strong> conductors supplying power to these loads.<br />
In real terms this involves:<br />
b setting up sub-distribution switchboards as close as possible to<br />
the barycenter <strong>of</strong> the groups <strong>of</strong> loads if they are localized,<br />
b setting up busbar trunking systems as close as possible to the<br />
barycenter <strong>of</strong> the groups <strong>of</strong> loads if they are distributed.<br />
The search for an optimal solution may lead to consider several<br />
clustering scenarios.<br />
In all cases, reducing the distance between the barycenter <strong>of</strong><br />
a group <strong>of</strong> loads and the equipment that supplies them power<br />
allows to reduce environmental impact.<br />
Fig. D27 : Environmental optimization : Objectives and Ressources.<br />
As an example figure D28 shows the impact <strong>of</strong> clustering circuits on reducing<br />
the distance between the barycenter <strong>of</strong> the loads <strong>of</strong> an <strong>installation</strong> and that <strong>of</strong> the<br />
sources considered (MLVS whose position is imposed). This example concerns a<br />
mineral water bottling plant for which:<br />
b the position <strong>of</strong> <strong>electrical</strong> equipment (MLVS) is imposed in the premises outside <strong>of</strong><br />
the process area for reasons <strong>of</strong> accessibility and atmosphere constraints,<br />
b the installed power is around 4 MVA.<br />
In solution No.1, the circuits are distributed for each workshop.<br />
In solution No. 2, the circuits are distributed by process functions (production lines).<br />
Barycenter<br />
line 2<br />
Workshop 2<br />
Workshop 2<br />
Barycenter<br />
Workshop 2<br />
Barycenter<br />
line 3<br />
Workshop 3<br />
Workshop 3<br />
Barycenter<br />
Workshop 3<br />
Barycenter<br />
line 3<br />
Storage<br />
Storage<br />
D27<br />
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D28<br />
© Schneider Electric - all rights reserved<br />
D - MV & LV architecture selection guide<br />
9 Recommendations for<br />
architecture optimization<br />
Without changing the layout <strong>of</strong> <strong>electrical</strong> equipment, the second solution allows us<br />
to achieve gains <strong>of</strong> around 15% on the weight <strong>of</strong> LV cables to be installed (gain on<br />
lengths) and a better uniformity <strong>of</strong> transformer power.<br />
To supplement the optimizations carried out in terms <strong>of</strong> architecture, the following<br />
points also contribute to the optimization:<br />
b the setting up <strong>of</strong> LV power factor correction to limit losses in the transformers and<br />
LV circuits if this compensation is distributed,<br />
b the use <strong>of</strong> low loss transformers,<br />
b the use <strong>of</strong> aluminum LV busbar trunking when possible, since natural resources <strong>of</strong><br />
this metal are greater.<br />
9.3 Preventive maintenance volume<br />
Recommendations for reducing the volume <strong>of</strong> preventive maintenance:<br />
b Use the same recommendations as for reducing the work site time,<br />
b Focus maintenance work on critical circuits,<br />
b Standardize the choice <strong>of</strong> equipment,<br />
b Use equipment <strong>design</strong>ed for severe atmospheres (requires less maintenance).<br />
9.4 Electrical power availability<br />
Recommendations for improving the <strong>electrical</strong> power availability:<br />
b Reduce the number <strong>of</strong> feeders per switchboard, in order to limit the effects <strong>of</strong> a<br />
possible failure <strong>of</strong> a switchboard,<br />
b Distributing circuits according to availability requirements,<br />
b Using equipment that is in line with requirements (see Service Index, 4.2),<br />
b Follow the selection guides proposed for steps 1 & 2 (see Fig. D3 page D5).<br />
Recommendations to increase the level <strong>of</strong> availability:<br />
b Change from a radial single feeder configuration to a two-pole configuration,<br />
b Change from a two-pole configuration to a double-ended configuration,<br />
b Change from a double-ended configuration to a uninterruptible configuration with a<br />
UPS unit and a Static Transfer Switch<br />
b Increase the level <strong>of</strong> maintenance (reducing the MTTR, increasing the MTBF)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
D - MV & LV architecture selection guide<br />
10 Glossary<br />
Architecture: choice <strong>of</strong> a single-line diagram and technological solutions, from<br />
connection to the utility network through to load power supply circuits.<br />
Main MV/LV distribution: Level upstream <strong>of</strong> the architecture, from connection<br />
to the network utility through to LV distribution equipment on the site (MLVS – or<br />
equivalent).<br />
MLVS – Main Low Voltage Switchboard: Main switchboard downstream <strong>of</strong> the<br />
MV/LV transformer, starting point <strong>of</strong> power distribution circuits in the <strong>installation</strong><br />
LV power distribution: intermediate level in the architecture, downstream <strong>of</strong> the<br />
main level through to the sub-distribution switchboards (spatial and functional<br />
distribution <strong>of</strong> <strong>electrical</strong> power in the circuits).<br />
LV terminal distribution: Downstream level <strong>of</strong> the architecture, downstream <strong>of</strong> the<br />
sub-distribution switchboards through to the loads. This level <strong>of</strong> distribution is not<br />
dealt with in this guide.<br />
Single-line diagram: general <strong>electrical</strong> schematic diagram to represent the main<br />
<strong>electrical</strong> equipment and their interconnection.<br />
MV substation, transformation substation: Enclosures grouping together MV<br />
equipment and/or MV/LV transformers. These enclosures can be shared or separate,<br />
according to the site layout, or the equipment technology. In certain countries, the<br />
MV substation is assimilated with the delivery substation.<br />
Technological solution: Resulting from the choice <strong>of</strong> technology for an <strong>installation</strong><br />
sub-assembly, from among the different products and equipment proposed by the<br />
manufacturer.<br />
Characteristics: Technical or environmental data relative to the <strong>installation</strong>, enabling<br />
the best-suited architecture to be selected.<br />
Criteria: Parameters for assessing the <strong>installation</strong>, enabling selection <strong>of</strong> the<br />
architecture that is the best-suited to the needs <strong>of</strong> the customer.<br />
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D30<br />
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D - MV & LV architecture selection guide<br />
11 ID-Spec s<strong>of</strong>tware<br />
ID-Spec is a new s<strong>of</strong>tware which aims at helping the <strong>design</strong>er to be more productive<br />
in draft <strong>design</strong> phase and argue easily his <strong>design</strong> decisions.<br />
It supports the <strong>design</strong>er in selecting the relevant single line diagram patterns for main<br />
distribution and sub distribution and in adapting these patterns to his project. It also<br />
supports the <strong>design</strong>er in equipment technology and rating selection. Its generates<br />
automatically the corresponding <strong>design</strong> specification documentation including single<br />
line diagram and its argument, list and specification <strong>of</strong> the corresponding equipment.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
D - MV & LV architecture selection guide<br />
12 Example: <strong>electrical</strong> <strong>installation</strong><br />
in a printworks<br />
12.1 Brief description<br />
Printing <strong>of</strong> personalized mailshots intended for mail order sales.<br />
12.2 Installation characteristics<br />
Characteristic Category<br />
Activity Mechanical<br />
Site topology single storey building,<br />
10000m² (8000m² dedicated to the process, 2000m² for<br />
ancillary areas)<br />
Layout latitude High<br />
Service reliability Standard<br />
Maintainability Standard<br />
Installation flexibility b No flexibility planned:<br />
v HVAC<br />
v Process utilities<br />
v Office power supply<br />
b Possible flexibility:<br />
v finishing, putting in envelopes<br />
v special machines, installed at a later date<br />
v rotary machines (uncertainty at the draft <strong>design</strong> stage)<br />
Power demand 3500kVA<br />
Load distribution Intermediate distribution<br />
Power interruptions sensitivity b Sheddable circuits:<br />
v <strong>of</strong>fices (apart from PC power sockets)<br />
v air conditioning, <strong>of</strong>fice heating<br />
v social premises<br />
v maintenance premises<br />
b long interruptions acceptable:<br />
v printing machines<br />
v workshop HVAC (hygrometric control)<br />
v Finishing, envelope filling<br />
v Process utilities (compressor, recycling <strong>of</strong> cooled water)<br />
b No interruptions acceptable:<br />
v servers, <strong>of</strong>fice PCs<br />
Disturbance sensitivity b Average sensitivity:<br />
v motors, lighting<br />
b High sensitivity:<br />
v IT<br />
Disturbance capability Non disturbing<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
No special precaution to be taken due to the connection to<br />
the EdF network (low level <strong>of</strong> disturbance)<br />
Other constraints b Building with lightning classification: lightning surge<br />
arresters installed<br />
b Power supply by overhead single feeder line<br />
12.3 Technological characteristics<br />
Criteria Category<br />
Atmosphere, environment b IP: standard (no dust, no water protection)<br />
b IK: standard (use <strong>of</strong> technical pits, dedicated premises)<br />
b °C: standard (temperature regulation)<br />
Service index 211<br />
Offer availability by country No problem (project carried out in France)<br />
Other criteria Nothing particular<br />
D31<br />
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D32<br />
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D - MV & LV architecture selection guide<br />
12 Example: <strong>electrical</strong> <strong>installation</strong><br />
in a printworks<br />
12.4 Architecture assessment criteria<br />
Criteria Category<br />
On-site work time Secondary<br />
Environmental impact Minimal: compliance with European standard regulations<br />
Preventive maintenance costs Standard<br />
Power supply availability Level I<br />
Step 1: Architecture fundamentals<br />
Choice Main criteria Solution<br />
Connection to upstream<br />
network<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Isolated site single branch circuit<br />
MV Circuits Layout + criticality single feeder<br />
Number <strong>of</strong> transformers Power > 2500kVA 2 x 2000kVA<br />
Number and distribution <strong>of</strong><br />
substations<br />
Surface area and power<br />
distribution<br />
MV Generator Site activity No<br />
MV<br />
LV<br />
MV<br />
LV<br />
2 possible solutions: 1<br />
substation or 2 substations<br />
b if 1 substations: NO link<br />
between MLVS<br />
b if 2 substations:<br />
interconnected switchboards<br />
MLVS 1 MLVS 2 MLVS 1 MLVS 2<br />
Fig. D29 : Two possible single-line diagrams<br />
MV<br />
LV<br />
Trunking<br />
MV<br />
LV
D - MV & LV architecture selection guide<br />
12 Example: <strong>electrical</strong> <strong>installation</strong><br />
in a printworks<br />
Step 2: Architecture details<br />
“1 substation” solution<br />
Choice Main criteria Solution<br />
Layout Atmospheric constraint Dedicated premises<br />
Centralized or decentralized<br />
layout<br />
Presence <strong>of</strong> back-up<br />
generator<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Uniform loads, distributed<br />
power, scalability possibilities<br />
Non-uniform loads, direct link<br />
from MLVS<br />
Criticality ≤ low<br />
Network availability: standard<br />
b Decentralized with busbar<br />
trunking:<br />
v finishing sector, envelope<br />
filling<br />
b Centralized with cables:<br />
v special machines,<br />
rotary machines, HVAC,<br />
process utilities, <strong>of</strong>fices<br />
(2 switchboards), <strong>of</strong>fice air<br />
conditioning, social premises,<br />
maintenance<br />
No back-up generator<br />
Presence <strong>of</strong> UPS Criticality UPS unit for servers and <strong>of</strong>fice<br />
PCs<br />
LV circuit configuration 2 transformers, possible<br />
partial redundancy<br />
MV<br />
MLVS 1 MLVS 2<br />
ASI<br />
Sheddable Offices<br />
Fig. D30 : Detailed single-line diagram (1 substation)<br />
LV<br />
HVAC<br />
MV<br />
LV<br />
b Two-pole, variant 2 ½ MLVS<br />
+ NO link (reduction <strong>of</strong> the Isc<br />
by MLVS, no redundancy<br />
b process (≤ weak)<br />
b sheddable circuit for noncritical<br />
loads<br />
Trunking<br />
Machines<br />
D33<br />
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D34<br />
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D - MV & LV architecture selection guide<br />
12 Example: <strong>electrical</strong> <strong>installation</strong><br />
in a printworks<br />
12.5 Choice <strong>of</strong> technological solutions:<br />
Choice Main criteria Solution<br />
MV/LV substation Atmosphere, environment indoor (dedicated premises)<br />
MV switchboard Offer availability by country SM6 (<strong>installation</strong> produced in<br />
France)<br />
Transformers Atmosphere, environment cast resin transfo (avoids<br />
constraints related to oil)<br />
LV switchboard Atmosphere, IS MLVS: Prisma + P<br />
Sub-distribution: Prisma +<br />
Busbar trunking Installed power to be<br />
supplied<br />
UPS units Installed power to be<br />
supplied, back-up time<br />
Power factor correction Installed power, presence <strong>of</strong><br />
harmonics<br />
“2 substation” solution<br />
Ditto apart from:<br />
LV circuit: 2 remote MLVS connected via busbar trunking<br />
MV<br />
LV<br />
Fig. D31 : Detailed single-line diagram (2 substations)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Canalis KS<br />
Galaxy PW<br />
MLVS 1 MLVS 2<br />
Sheddable<br />
ASI<br />
Offices<br />
LV, standard, automatic<br />
(Average Q, ease <strong>of</strong><br />
<strong>installation</strong>)<br />
Trunking Trunking<br />
HVAC<br />
MV<br />
LV<br />
Machines
2<br />
3<br />
<strong>Chapter</strong> E<br />
LV Distribution<br />
Contents<br />
Earthing schemes E2<br />
1.1 Earthing connections E2<br />
1.2 Definition <strong>of</strong> standardised earthing schemes E3<br />
1.3 Characteristics <strong>of</strong> TT, TN and IT systems E6<br />
1.4 Selection criteria for the TT, TN and IT systems E8<br />
1.5 Choice <strong>of</strong> earthing method - implementation E10<br />
1.6 Installation and measurements <strong>of</strong> earth electrodes E11<br />
The <strong>installation</strong> system E 5<br />
2.1 Distribution boards E15<br />
2.2 Cables and busways E18<br />
External influences (IEC 60364-5-5 ) E25<br />
3.1 Definition and reference standards E25<br />
3.2 Classification E25<br />
3.3 List <strong>of</strong> external influences E25<br />
3.4 Protection provided for enclosed equipment: codes IP and IK E28<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
E<br />
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© Schneider Electric - all rights reserved<br />
E2<br />
E - Distribution in low-voltage <strong>installation</strong>s<br />
In a building, the connection <strong>of</strong> all metal parts<br />
<strong>of</strong> the building and all exposed conductive parts<br />
<strong>of</strong> <strong>electrical</strong> equipment to an earth electrode<br />
prevents the appearance <strong>of</strong> dangerously high<br />
voltages between any two simultaneously<br />
accessible metal parts<br />
4<br />
Extraneous<br />
conductive<br />
parts<br />
4<br />
Heating<br />
Water<br />
Branched<br />
protective<br />
conductors<br />
to individual<br />
consumers<br />
Gas<br />
5<br />
1<br />
5<br />
5<br />
Fig. E1 : An example <strong>of</strong> a block <strong>of</strong> flats in which the main<br />
earthing terminal (6) provides the main equipotential connection;<br />
the removable link (7) allows an earth-electrode-resistance<br />
check<br />
2<br />
7<br />
3<br />
3<br />
3<br />
6<br />
Main<br />
protective<br />
conductor<br />
Earthing schemes<br />
. Earthing connections<br />
Definitions<br />
National and international standards (IEC 60364) clearly define the various elements<br />
<strong>of</strong> earthing connections. The following terms are commonly used in industry and in<br />
the literature. Bracketed numbers refer to Figure E :<br />
b Earth electrode (1): A conductor or group <strong>of</strong> conductors in intimate contact with,<br />
and providing an <strong>electrical</strong> connection with Earth (cf details in section 1.6 <strong>of</strong> <strong>Chapter</strong> E.)<br />
b Earth: The conductive mass <strong>of</strong> the Earth, whose electric potential at any point is<br />
conventionally taken as zero<br />
b Electrically independent earth electrodes: Earth electrodes located at such a<br />
distance from one another that the maximum current likely to flow through one <strong>of</strong><br />
them does not significantly affect the potential <strong>of</strong> the other(s)<br />
b Earth electrode resistance: The contact resistance <strong>of</strong> an earth electrode with the<br />
Earth<br />
b Earthing conductor (2): A protective conductor connecting the main earthing<br />
terminal (6) <strong>of</strong> an <strong>installation</strong> to an earth electrode (1) or to other means <strong>of</strong> earthing<br />
(e.g. TN systems);<br />
b Exposed-conductive-part: A conductive part <strong>of</strong> equipment which can be touched<br />
and which is not a live part, but which may become live under fault conditions<br />
b Protective conductor (3): A conductor used for some measures <strong>of</strong> protection against<br />
electric shock and intended for connecting together any <strong>of</strong> the following parts:<br />
v Exposed-conductive-parts<br />
v Extraneous-conductive-parts<br />
v The main earthing terminal<br />
v Earth electrode(s)<br />
v The earthed point <strong>of</strong> the source or an artificial neutral<br />
b Extraneous-conductive-part: A conductive part liable to introduce a potential,<br />
generally earth potential, and not forming part <strong>of</strong> the <strong>electrical</strong> <strong>installation</strong> (4).<br />
For example:<br />
v Non-insulated floors or walls, metal framework <strong>of</strong> buildings<br />
v Metal conduits and pipework (not part <strong>of</strong> the <strong>electrical</strong> <strong>installation</strong>) for water, gas,<br />
heating, compressed-air, etc. and metal materials associated with them<br />
b Bonding conductor (5): A protective conductor providing equipotential bonding<br />
b Main earthing terminal (6): The terminal or bar provided for the connection <strong>of</strong><br />
protective conductors, including equipotential bonding conductors, and conductors<br />
for functional earthing, if any, to the means <strong>of</strong> earthing.<br />
Connections<br />
The main equipotential bonding system<br />
The bonding is carried out by protective conductors and the aim is to ensure that,<br />
in the event <strong>of</strong> an incoming extraneous conductor (such as a gas pipe, etc.) being<br />
raised to some potential due to a fault external to the building, no difference <strong>of</strong><br />
potential can occur between extraneous-conductive-parts within the <strong>installation</strong>.<br />
The bonding must be effected as close as possible to the point(s) <strong>of</strong> entry into the<br />
building, and be connected to the main earthing terminal (6).<br />
However, connections to earth <strong>of</strong> metallic sheaths <strong>of</strong> communications cables require<br />
the authorisation <strong>of</strong> the owners <strong>of</strong> the cables.<br />
Supplementary equipotential connections<br />
These connections are intended to connect all exposed-conductive-parts and all<br />
extraneous-conductive-parts simultaneously accessible, when correct conditions<br />
for protection have not been met, i.e. the original bonding conductors present an<br />
unacceptably high resistance.<br />
Connection <strong>of</strong> exposed-conductive-parts to the earth electrode(s)<br />
The connection is made by protective conductors with the object <strong>of</strong> providing a lowresistance<br />
path for fault currents flowing to earth.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
E - Distribution in low-voltage <strong>installation</strong>s<br />
The different earthing schemes (<strong>of</strong>ten referred<br />
to as the type <strong>of</strong> power system or system<br />
earthing arrangements) described characterise<br />
the method <strong>of</strong> earthing the <strong>installation</strong><br />
downstream <strong>of</strong> the secondary winding <strong>of</strong> a<br />
MV/LV transformer and the means used for<br />
earthing the exposed conductive-parts <strong>of</strong> the<br />
LV <strong>installation</strong> supplied from it<br />
Earthing schemes<br />
Components (see Fig. E2)<br />
Effective connection <strong>of</strong> all accessible metal fixtures and all exposed-conductive-parts<br />
<strong>of</strong> <strong>electrical</strong> appliances and equipment, is essential for effective protection against<br />
electric shocks.<br />
Component parts to consider:<br />
as exposed-conductive-parts as extraneous-conductive-parts<br />
Cableways Elements used in building construction<br />
b Conduits b Metal or reinforced concrete (RC):<br />
b Impregnated-paper-insulated lead-covered v Steel-framed structure<br />
cable, armoured or unarmoured v Reinforcement rods<br />
b Mineral insulated metal-sheathed cable v Prefabricated RC panels<br />
(pyrotenax, etc.) b Surface finishes:<br />
Switchgear<br />
b cradle <strong>of</strong> withdrawable switchgear<br />
Appliances<br />
b Exposed metal parts <strong>of</strong> class 1 insulated<br />
appliances<br />
v Floors and walls in reinforced concrete<br />
without further surface treatment<br />
v<br />
Tiled surface<br />
b<br />
Metallic covering:<br />
v<br />
Metallic wall covering<br />
Non-<strong>electrical</strong> elements Building services elements other than <strong>electrical</strong><br />
b metallic fittings associated with cableways b Metal pipes, conduits, trunking, etc. for gas,<br />
(cable trays, cable ladders, etc.) water and heating systems, etc.<br />
b Metal objects: b Related metal components (furnaces, tanks,<br />
v Close to aerial conductors or to busbars reservoirs, radiators)<br />
v In contact with <strong>electrical</strong> equipment. b Metallic fittings in wash rooms, bathrooms,<br />
toilets, etc.<br />
b Metallised papers<br />
Component parts not to be considered:<br />
as exposed-conductive-parts as extraneous-conductive-parts<br />
Diverse service channels, ducts, etc. b Wooden-block floors<br />
b Conduits made <strong>of</strong> insulating material b Rubber-covered or linoleum-covered floors<br />
b Mouldings in wood or other insulating b Dry plaster-block partition<br />
material b Brick walls<br />
b Conductors and cables without metallic sheaths b Carpets and wall-to-wall carpeting<br />
Switchgear<br />
b Enclosures made <strong>of</strong> insulating material<br />
Appliances<br />
b All appliances having class II insulation<br />
regardless <strong>of</strong> the type <strong>of</strong> exterior envelope<br />
Fig. E2 : List <strong>of</strong> exposed-conductive-parts and extraneous-conductive-parts<br />
.2 Definition <strong>of</strong> standardised earthing schemes<br />
The choice <strong>of</strong> these methods governs the measures necessary for protection against<br />
indirect-contact hazards.<br />
The earthing system qualifies three originally independent choices made by the<br />
<strong>design</strong>er <strong>of</strong> an <strong>electrical</strong> distribution system or <strong>installation</strong>:<br />
b The type <strong>of</strong> connection <strong>of</strong> the <strong>electrical</strong> system (that is generally <strong>of</strong> the neutral<br />
conductor) and <strong>of</strong> the exposed parts to earth electrode(s)<br />
b A separate protective conductor or protective conductor and neutral conductor<br />
being a single conductor<br />
b The use <strong>of</strong> earth fault protection <strong>of</strong> overcurrent protective switchgear which clear<br />
only relatively high fault currents or the use <strong>of</strong> additional relays able to detect and<br />
clear small insulation fault currents to earth<br />
In practice, these choices have been grouped and standardised as explained below.<br />
Each <strong>of</strong> these choices provides standardised earthing systems with three<br />
advantages and drawbacks:<br />
b Connection <strong>of</strong> the exposed conductive parts <strong>of</strong> the equipment and <strong>of</strong> the neutral<br />
conductor to the PE conductor results in equipotentiality and lower overvoltages but<br />
increases earth fault currents<br />
b A separate protective conductor is costly even if it has a small cross-sectional area<br />
but it is much more unlikely to be polluted by voltage drops and harmonics, etc. than a<br />
neutral conductor is. Leakage currents are also avoided in extraneous conductive parts<br />
b Installation <strong>of</strong> residual current protective relays or insulation monitoring devices are<br />
much more sensitive and permits in many circumstances to clear faults before heavy<br />
damage occurs (motors, fires, electrocution). The protection <strong>of</strong>fered is in addition<br />
independent with respect to changes in an existing <strong>installation</strong><br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
E3<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
E4<br />
E - Distribution in low-voltage <strong>installation</strong>s<br />
Rn<br />
Neutral<br />
Earth<br />
Fig. E3 : TT System<br />
Rn<br />
Neutral<br />
Earth<br />
Fig. E4 : TN-C system<br />
Rn<br />
Fig. E5 : TN-S system<br />
Fig. E6 : TN-C-S system<br />
Exposed conductive parts<br />
Earth<br />
Exposed conductive parts<br />
Neutral<br />
PEN<br />
L1<br />
L2<br />
L3<br />
N<br />
PE<br />
L1<br />
L2<br />
L3<br />
PEN<br />
L1<br />
L2<br />
L3<br />
N<br />
PE<br />
TT system (earthed neutral) (see Fig. E3)<br />
One point at the supply source is connected directly to earth. All exposed- and<br />
extraneous-conductive-parts are connected to a separate earth electrode at the<br />
<strong>installation</strong>. This electrode may or may not be <strong>electrical</strong>ly independent <strong>of</strong> the source<br />
electrode. The two zones <strong>of</strong> influence may overlap without affecting the operation <strong>of</strong><br />
protective devices.<br />
TN systems (exposed conductive parts connected to the<br />
neutral)<br />
The source is earthed as for the TT system (above). In the <strong>installation</strong>, all exposed-<br />
and extraneous-conductive-parts are connected to the neutral conductor. The several<br />
versions <strong>of</strong> TN systems are shown below.<br />
TN-C system (see Fig. E4)<br />
The neutral conductor is also used as a protective conductor and is referred to as<br />
a PEN (Protective Earth and Neutral) conductor. This system is not permitted for<br />
conductors <strong>of</strong> less than 10 mm 2 or for portable equipment.<br />
The TN-C system requires an effective equipotential environment within the<br />
<strong>installation</strong> with dispersed earth electrodes spaced as regularly as possible since<br />
the PEN conductor is both the neutral conductor and at the same time carries phase<br />
unbalance currents as well as 3 rd order harmonic currents (and their multiples).<br />
The PEN conductor must therefore be connected to a number <strong>of</strong> earth electrodes in<br />
the <strong>installation</strong>.<br />
Caution: In the TN-C system, the “protective conductor” function has priority over<br />
the “neutral function”. In particular, a PEN conductor must always be connected to<br />
the earthing terminal <strong>of</strong> a load and a jumper is used to connect this terminal to the<br />
neutral terminal.<br />
TN-S system (see Fig. E5)<br />
The TN-S system (5 wires) is obligatory for circuits with cross-sectional areas less<br />
than 10 mm 2 for portable equipment.<br />
The protective conductor and the neutral conductor are separate. On underground<br />
cable systems where lead-sheathed cables exist, the protective conductor is<br />
generally the lead sheath. The use <strong>of</strong> separate PE and N conductors (5 wires)<br />
is obligatory for circuits with cross-sectional areas less than 10 mm 2 for portable<br />
equipment.<br />
TN-C-S system (see Fig. E6 below and Fig. E7 next page)<br />
The TN-C and TN-S systems can be used in the same <strong>installation</strong>. In the TN-C-S<br />
system, the TN-C (4 wires) system must never be used downstream <strong>of</strong> the TN-S<br />
(5 wires) system, since any accidental interruption in the neutral on the upstream<br />
part would lead to an interruption in the protective conductor in the downstream part<br />
and therefore a danger.<br />
PE<br />
Earthing schemes<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
5 x 50 mm 2<br />
Bad Bad<br />
L1<br />
L2<br />
L3<br />
N<br />
PE<br />
16 mm2 6 mm2 16 mm2 16 mm2<br />
PEN<br />
TN-C scheme not permitted<br />
downstream <strong>of</strong> TN-S scheme
E - Distribution in low-voltage <strong>installation</strong>s<br />
Neutral<br />
Isolated or<br />
impedance-earthed<br />
Fig. E8 : IT system (isolated neutral)<br />
MV/LV<br />
C1<br />
Fig. E9 : IT system (isolated neutral)<br />
Exposed conductive parts<br />
Earth<br />
C2<br />
MV/LV<br />
Zct<br />
C3<br />
R1<br />
R2<br />
L1<br />
L2<br />
L3<br />
N<br />
PE<br />
R3<br />
Earthing schemes<br />
Fig. E7 : Connection <strong>of</strong> the PEN conductor in the TN-C system<br />
IT system (isolated or impedance-earthed neutral)<br />
IT system (isolated neutral)<br />
No intentional connection is made between the neutral point <strong>of</strong> the supply source<br />
and earth (see Fig. E8).<br />
Exposed- and extraneous-conductive-parts <strong>of</strong> the <strong>installation</strong> are connected to an<br />
earth electrode.<br />
In practice all circuits have a leakage impedance to earth, since no insulation<br />
is perfect. In parallel with this (distributed) resistive leakage path, there is the<br />
distributed capacitive current path, the two paths together constituting the normal<br />
leakage impedance to earth (see Fig. E9).<br />
Example (see Fig. E 0)<br />
In a LV 3-phase 3-wire system, 1 km <strong>of</strong> cable will have a leakage impedance due to<br />
C1, C2, C3 and R1, R2 and R3 equivalent to a neutral earth impedance Zct <strong>of</strong> 3,000<br />
to 4,000 Ω, without counting the filtering capacitances <strong>of</strong> electronic devices.<br />
IT system (impedance-earthed neutral)<br />
An impedance Zs (in the order <strong>of</strong> 1,000 to 2,000 Ω) is connected permanently<br />
between the neutral point <strong>of</strong> the transformer LV winding and earth (see Fig. E ).<br />
All exposed- and extraneous-conductive-parts are connected to an earth electrode.<br />
The reasons for this form <strong>of</strong> power-source earthing are to fix the potential <strong>of</strong> a small<br />
network with respect to earth (Zs is small compared to the leakage impedance) and to<br />
reduce the level <strong>of</strong> overvoltages, such as transmitted surges from the MV windings,<br />
static charges, etc. with respect to earth. It has, however, the effect <strong>of</strong> slightly<br />
increasing the first-fault current level.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
4 x 95 mm 2<br />
Correct Incorrect Correct Incorrect<br />
PEN connected to the neutral<br />
terminal is prohibited<br />
Fig. E10 : Impedance equivalent to leakage impedances in an<br />
IT system Fig. E11 : IT system (impedance-earthed neutral)<br />
L1<br />
L2<br />
L3<br />
PEN<br />
16 mm2 10 mm2 6 mm2 6 mm2 PEN<br />
PEN<br />
N<br />
MV/LV<br />
Zs<br />
S < 10 mm 2<br />
TNC prohibited<br />
E5<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
E6<br />
E - Distribution in low-voltage <strong>installation</strong>s<br />
The TT system:<br />
b Technique for the protection <strong>of</strong> persons: the<br />
exposed conductive parts are earthed and<br />
residual current devices (RCDs) are used<br />
b Operating technique: interruption for the first<br />
insulation fault<br />
The TN system:<br />
b Technique for the protection <strong>of</strong> persons:<br />
v Interconnection and earthing <strong>of</strong> exposed<br />
conductive parts and the neutral are mandatory<br />
v Interruption for the first fault using overcurrent<br />
protection (circuit-breakers or fuses)<br />
b Operating technique: interruption for the first<br />
insulation fault<br />
Earthing schemes<br />
.3 Characteristics <strong>of</strong> TT, TN and IT systems<br />
TT system (see Fig. E 2)<br />
Fig. E12 : TT system<br />
Note: If the exposed conductive parts are earthed at a number <strong>of</strong> points, an RCD<br />
must be installed for each set <strong>of</strong> circuits connected to a given earth electrode.<br />
Main characteristics<br />
b Simplest solution to <strong>design</strong> and install. Used in <strong>installation</strong>s supplied directly by the<br />
public LV distribution network.<br />
b Does not require continuous monitoring during operation (a periodic check on the<br />
RCDs may be necessary).<br />
b Protection is ensured by special devices, the residual current devices (RCD), which<br />
also prevent the risk <strong>of</strong> fire when they are set to y 500 mA.<br />
b Each insulation fault results in an interruption in the supply <strong>of</strong> power, however the<br />
outage is limited to the faulty circuit by installing the RCDs in series (selective RCDs)<br />
or in parallel (circuit selection).<br />
b Loads or parts <strong>of</strong> the <strong>installation</strong> which, during normal operation, cause high leakage<br />
currents, require special measures to avoid nuisance tripping, i.e. supply the loads<br />
with a separation transformer or use specific RCDs (see section 5.1 in chapter F).<br />
TN system (see Fig. E 3 and Fig. E 4 )<br />
Fig. E13 : TN-C system<br />
Fig. E14 : TN-S system<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
PEN<br />
N<br />
PE
E - Distribution in low-voltage <strong>installation</strong>s<br />
IT system:<br />
b Protection technique:<br />
v Interconnection and earthing <strong>of</strong> exposed<br />
conductive parts<br />
v Indication <strong>of</strong> the first fault by an insulation<br />
monitoring device (IMD)<br />
v Interruption for the second fault using<br />
overcurrent protection (circuit-breakers or fuses)<br />
b Operating technique:<br />
v Monitoring <strong>of</strong> the first insulation fault<br />
v Mandatory location and clearing <strong>of</strong> the fault<br />
v Interruption for two simultaneous insulation<br />
faults<br />
Earthing schemes<br />
Main characteristics<br />
b <strong>General</strong>ly speaking, the TN system:<br />
v requires the <strong>installation</strong> <strong>of</strong> earth electrodes at regular intervals throughout the<br />
<strong>installation</strong><br />
v Requires that the initial check on effective tripping for the first insulation fault<br />
be carried out by calculations during the <strong>design</strong> stage, followed by mandatory<br />
measurements to confirm tripping during commissioning<br />
v Requires that any modification or extension be <strong>design</strong>ed and carried out by a<br />
qualified electrician<br />
v May result, in the case <strong>of</strong> insulation faults, in greater damage to the windings <strong>of</strong><br />
rotating machines<br />
v May, on premises with a risk <strong>of</strong> fire, represent a greater danger due to the higher<br />
fault currents<br />
b In addition, the TN-C system:<br />
v At first glance, would appear to be less expensive (elimination <strong>of</strong> a device pole and<br />
<strong>of</strong> a conductor)<br />
v Requires the use <strong>of</strong> fixed and rigid conductors<br />
v Is forbidden in certain cases:<br />
- Premises with a risk <strong>of</strong> fire<br />
- For computer equipment (presence <strong>of</strong> harmonic currents in the neutral)<br />
b In addition, the TN-S system:<br />
v May be used even with flexible conductors and small conduits<br />
v Due to the separation <strong>of</strong> the neutral and the protection conductor, provides a clean<br />
PE (computer systems and premises with special risks)<br />
IT system (see Fig. E 5)<br />
Cardew<br />
Fig. E15 : IT system<br />
IMD<br />
Main characteristics<br />
b Solution <strong>of</strong>fering the best continuity <strong>of</strong> service during operation<br />
b Indication <strong>of</strong> the first insulation fault, followed by mandatory location and clearing,<br />
ensures systematic prevention <strong>of</strong> supply outages<br />
b <strong>General</strong>ly used in <strong>installation</strong>s supplied by a private MV/LV or LV/LV transformer<br />
b Requires maintenance personnel for monitoring and operation<br />
b Requires a high level <strong>of</strong> insulation in the network (implies breaking up the network<br />
if it is very large and the use <strong>of</strong> circuit-separation transformers to supply loads with<br />
high leakage currents)<br />
b The check on effective tripping for two simultaneous faults must be carried out by<br />
calculations during the <strong>design</strong> stage, followed by mandatory measurements during<br />
commissioning on each group <strong>of</strong> interconnected exposed conductive parts<br />
b Protection <strong>of</strong> the neutral conductor must be ensured as indicated in section 7.2 <strong>of</strong><br />
<strong>Chapter</strong> G<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
E7<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
E8<br />
E - Distribution in low-voltage <strong>installation</strong>s<br />
Selection does not depend on safety criteria.<br />
The three systems are equivalent in terms<br />
<strong>of</strong> protection <strong>of</strong> persons if all <strong>installation</strong> and<br />
operating <strong>rules</strong> are correctly followed.<br />
The selection criteria for the best system(s)<br />
depend on the regulatory requirements,<br />
the required continuity <strong>of</strong> service, operating<br />
conditions and the types <strong>of</strong> network and loads.<br />
Fig. E16 : Comparison <strong>of</strong> system earthing arrangements<br />
Earthing schemes<br />
.4 Selection criteria for the TT, TN and IT systems<br />
In terms <strong>of</strong> the protection <strong>of</strong> persons, the three system earthing arrangements<br />
(SEA) are equivalent if all <strong>installation</strong> and operating <strong>rules</strong> are correctly followed.<br />
Consequently, selection does not depend on safety criteria.<br />
It is by combining all requirements in terms <strong>of</strong> regulations, continuity <strong>of</strong> service,<br />
operating conditions and the types <strong>of</strong> network and loads that it is possible to<br />
determine the best system(s) (see Fig. E 6).<br />
Selection is determined by the following factors:<br />
b Above all, the applicable regulations which in some cases impose certain types <strong>of</strong><br />
SEA<br />
b Secondly, the decision <strong>of</strong> the owner if supply is via a private MV/LV transformer<br />
(MV subscription) or the owner has a private energy source (or a separate-winding<br />
transformer)<br />
If the owner effectively has a choice, the decision on the SEA is taken following<br />
discussions with the network <strong>design</strong>er (<strong>design</strong> <strong>of</strong>fice, contractor)<br />
The discussions must cover:<br />
b First <strong>of</strong> all, the operating requirements (the required level <strong>of</strong> continuity <strong>of</strong> service)<br />
and the operating conditions (maintenance ensured by <strong>electrical</strong> personnel or not,<br />
in-house personnel or outsourced, etc.)<br />
b Secondly, the particular characteristics <strong>of</strong> the network and the loads<br />
(see Fig. E 7 next page)<br />
TT TN-S TN-C IT IT2 Comments<br />
Electrical characteristics<br />
Fault current - - - - - + - - Only the IT system <strong>of</strong>fers virtually negligible first-fault currents<br />
Fault voltage - - - + - In the IT system, the touch voltage is very low for the first fault,<br />
but is considerable for the second<br />
Touch voltage<br />
Protection<br />
+/- - - - + - In the TT system, the touch voltage is very low if system is<br />
equipotential, otherwise it is high<br />
Protection <strong>of</strong> persons against indirect contact + + + + + All SEAs (system earthing arrangement) are equivalent,<br />
if the <strong>rules</strong> are followed<br />
Protection <strong>of</strong> persons with emergency + - - + - Systems where protection is ensured by RCDs are not sensitive<br />
generating sets to a change in the internal impedance <strong>of</strong> the source<br />
Protection against fire (with an RCD) + + Not + + All SEAs in which RCDs can be used are equivalent.<br />
allowed The TN-C system is forbidden on premises where there is a risk <strong>of</strong> fire<br />
Overvoltages<br />
Continuous overvoltage + + + - + A phase-to-earth overvoltage is continuous in the IT system<br />
if there is a first insulation fault<br />
Transient overvoltage + - - + - Systems with high fault currents may cause transient overvoltages<br />
Overvoltage if transformer breakdown - + + + + In the TT system, there is a voltage imbalance between<br />
(primary/secondary)<br />
Electromagnetic compatibility<br />
the different earth electrodes. The other systems are interconnected<br />
to a single earth electrode<br />
Immunity to nearby lightning strikes - + + + + In the TT system, there may be voltage imbalances between<br />
the earth electrodes. In the TT system, there is a significant current<br />
loop between the two separate earth electrodes<br />
Immunity to lightning strikes on MV lines - - - - - All SEAs are equivalent when a MV line takes a direct lightning strike<br />
Continuous emission <strong>of</strong> an + + - + + Connection <strong>of</strong> the PEN to the metal structures <strong>of</strong> the building is<br />
electromagnetic field conducive to the continuous generation <strong>of</strong> electromagnetic fields<br />
Transient non-equipotentiality <strong>of</strong> the PE<br />
Continuity <strong>of</strong> service<br />
+ - - + - The PE is no longer equipotential if there is a high fault current<br />
Interruption for first fault - - - + + Only the IT system avoids tripping for the first insulation fault<br />
Voltage dip during insulation fault + - - + - The TN-S, TNC and IT (2nd Installation<br />
fault) systems generate high fault<br />
currents which may cause phase voltage dips<br />
Special devices - + + - - The TT system requires the use <strong>of</strong> RCDs. The IT system requires<br />
the use <strong>of</strong> IMDs<br />
Number <strong>of</strong> earth electrodes - + + -/+ -/+ The TT system requires two distinct earth electrodes. The IT system<br />
<strong>of</strong>fers a choice between one or two earth electrodes<br />
Number <strong>of</strong> cables<br />
Maintenance<br />
- - + - - Only the TN-C system <strong>of</strong>fers, in certain cases, a reduction in<br />
the number <strong>of</strong> cables<br />
Cost <strong>of</strong> repairs - - - - - - - - The cost <strong>of</strong> repairs depends on the damage caused by<br />
the amplitude <strong>of</strong> the fault currents<br />
Installation damage + - - ++ - Systems causing high fault currents require a check on<br />
the <strong>installation</strong> after clearing the fault<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
E - Distribution in low-voltage <strong>installation</strong>s<br />
Type <strong>of</strong> network Advised Possible Not advised<br />
Very large network with high-quality earth electrodes TT, TN, IT (1)<br />
for exposed conductive parts (10 Ω max.) or mixed<br />
Very large network with low-quality earth electrodes TN TN-S IT (1)<br />
for exposed conductive parts (> 30 Ω) TN-C<br />
Disturbed area (storms) TN TT IT (2)<br />
(e.g. television or radio transmitter)<br />
Network with high leakage currents (> 500 mA) TN (4) IT (4)<br />
Network with outdoor overhead lines TT<br />
TT (3) (4)<br />
(5) TN (5) (6) IT (6)<br />
Emergency standby generator set IT TT TN (7)<br />
Type <strong>of</strong> loads<br />
Loads sensitive to high fault currents (motors, etc.) IT TT TN (8)<br />
Loads with a low insulation level (electric furnaces, TN (9) TT (9) IT<br />
welding machines, heating elements, immersion heaters,<br />
equipment in large kitchens)<br />
Numerous phase-neutral single-phase loads TT (10) IT (10)<br />
(mobile, semi-fixed, portable) TN-S TN-C (10)<br />
Loads with sizeable risks (hoists, conveyers, etc.) TN (11) TT (11) IT (11)<br />
Numerous auxiliaries (machine tools) TN-S TN-C TT (12)<br />
(1) When the SEA is not imposed by regulations, it is selected according to the level <strong>of</strong> operating characteristics (continuity <strong>of</strong> service that is<br />
mandatory for safety reasons or desired to enhance productivity, etc.)<br />
Whatever the SEA, the probability <strong>of</strong> an insulation failure increases with the length <strong>of</strong> the network. It may be a good idea to break up the<br />
network, which facilitates fault location and makes it possible to implement the system advised above for each type <strong>of</strong> application.<br />
(2) The risk <strong>of</strong> flashover on the surge limiter turns the isolated neutral into an earthed neutral. These risks are high for regions with frequent<br />
thunder storms or <strong>installation</strong>s supplied by overhead lines. If the IT system is selected to ensure a higher level <strong>of</strong> continuity <strong>of</strong> service, the<br />
system <strong>design</strong>er must precisely calculate the tripping conditions for a second fault.<br />
(3) Risk <strong>of</strong> RCD nuisance tripping.<br />
(4) Whatever the SEA, the ideal solution is to isolate the disturbing section if it can be easily identified.<br />
(5) Risks <strong>of</strong> phase-to-earth faults affecting equipotentiality.<br />
(6) Insulation is uncertain due to humidity and conducting dust.<br />
(7) The TN system is not advised due to the risk <strong>of</strong> damage to the generator in the case <strong>of</strong> an internal fault. What is more, when generator sets<br />
supply safety equipment, the system must not trip for the first fault.<br />
(8) The phase-to-earth current may be several times higher than In, with the risk <strong>of</strong> damaging or accelerating the ageing <strong>of</strong> motor windings, or <strong>of</strong><br />
destroying magnetic circuits.<br />
(9) To combine continuity <strong>of</strong> service and safety, it is necessary and highly advised, whatever the SEA, to separate these loads from the rest <strong>of</strong><br />
the <strong>installation</strong> (transformers with local neutral connection).<br />
(10) When load equipment quality is not a <strong>design</strong> priority, there is a risk that the insulation resistance will fall rapidly. The TT system with RCDs<br />
is the best means to avoid problems.<br />
(11) The mobility <strong>of</strong> this type <strong>of</strong> load causes frequent faults (sliding contact for bonding <strong>of</strong> exposed conductive parts) that must be countered.<br />
Whatever the SEA, it is advised to supply these circuits using transformers with a local neutral connection.<br />
(12) Requires the use <strong>of</strong> transformers with a local TN system to avoid operating risks and nuisance tripping at the first fault (TT) or a double fault (IT).<br />
(12 bis) With a double break in the control circuit.<br />
(13) Excessive limitation <strong>of</strong> the phase-to-neutral current due to the high value <strong>of</strong> the zero-phase impedance (at least 4 to 5 times the direct<br />
impedance). This system must be replaced by a star-delta arrangement.<br />
(14) The high fault currents make the TN system dangerous. The TN-C system is forbidden.<br />
(15) Whatever the system, the RCD must be set to Δn y 500 mA.<br />
(16) An <strong>installation</strong> supplied with LV energy must use the TT system. Maintaining this SEA means the least amount <strong>of</strong> modifications on the<br />
existing network (no cables to be run, no protection devices to be modified).<br />
(17) Possible without highly competent maintenance personnel.<br />
(18) This type <strong>of</strong> <strong>installation</strong> requires particular attention in maintaining safety. The absence <strong>of</strong> preventive measures in the TN system means<br />
highly qualified personnel are required to ensure safety over time.<br />
(19) The risks <strong>of</strong> breaks in conductors (supply, protection) may cause the loss <strong>of</strong> equipotentiality for exposed conductive parts. A TT system or a<br />
TN-S system with 30 mA RCDs is advised and is <strong>of</strong>ten mandatory. The IT system may be used in very specific cases.<br />
(20) This solution avoids nuisance tripping for unexpected earth leakage.<br />
Fig. E17 : Influence <strong>of</strong> networks and loads on the selection <strong>of</strong> system earthing arrangements<br />
Earthing schemes<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
IT (12 bis)<br />
Miscellaneous<br />
Supply via star-star connected power transformer (13) TT IT IT (13)<br />
without neutral with neutral<br />
Premises with risk <strong>of</strong> fire IT (15) TN-S (15) TN-C (14)<br />
Increase in power level <strong>of</strong> LV utility subscription, TT (16)<br />
requiring a private substation<br />
Installation with frequent modifications TT (17) TN (18)<br />
IT (18)<br />
Installation where the continuity <strong>of</strong> earth circuits is uncertain TT (19) TN-S TN-C<br />
(work sites, old <strong>installation</strong>s) IT (19)<br />
Electronic equipment (computers, PLCs) TN-S TT TN-C<br />
Machine control-monitoring network, PLC sensors and actuators IT (20) LV<br />
MV/LV<br />
TN-S, TT<br />
TT (15)<br />
E9<br />
© Schneider Electric - all rights reserved
E 0<br />
© Schneider Electric - all rights reserved<br />
E - Distribution in low-voltage <strong>installation</strong>s<br />
Earthing schemes<br />
.5 Choice <strong>of</strong> earthing method - implementation<br />
After consulting applicable regulations, Figures E16 and E17 can be used as an aid<br />
in deciding on divisions and possible galvanic isolation <strong>of</strong> appropriate sections <strong>of</strong> a<br />
proposed <strong>installation</strong>.<br />
Division <strong>of</strong> source<br />
This technique concerns the use <strong>of</strong> several transformers instead <strong>of</strong> employing one<br />
high-rated unit. In this way, a load that is a source <strong>of</strong> network disturbances (large<br />
motors, furnaces, etc.) can be supplied by its own transformer.<br />
The quality and continuity <strong>of</strong> supply to the whole <strong>installation</strong> are thereby improved.<br />
The cost <strong>of</strong> switchgear is reduced (short-circuit current level is lower).<br />
The cost-effectiveness <strong>of</strong> separate transformers must be determined on a case by<br />
case basis.<br />
Network islands<br />
The creation <strong>of</strong> galvanically-separated “islands” by means <strong>of</strong> LV/LV transformers<br />
makes it possible to optimise the choice <strong>of</strong> earthing methods to meet specific<br />
requirements (see Fig. E 8 and Fig. E 9 ).<br />
Fig. E18 : TN-S island within an IT system<br />
TN-S system<br />
Fig. E19 : IT islands within a TN-S system<br />
Conclusion<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
MV/LV<br />
IT system<br />
MV/LV TN-S<br />
LV/LV<br />
Hospital<br />
LV/LV<br />
IMD<br />
TN-S system<br />
LV/LV<br />
IMD IMD<br />
IT IT<br />
Operating room<br />
The optimisation <strong>of</strong> the performance <strong>of</strong> the whole <strong>installation</strong> governs the choice <strong>of</strong><br />
earthing system.<br />
Including:<br />
b Initial investments, and<br />
b Future operational expenditures, hard to assess, that can arise from insufficient<br />
reliability, quality <strong>of</strong> equipment, safety, continuity <strong>of</strong> service, etc.<br />
An ideal structure would comprise normal power supply sources, local reserve<br />
power supply sources (see section 1.4 <strong>of</strong> <strong>Chapter</strong> E) and the appropriate earthing<br />
arrangements.
E - Distribution in low-voltage <strong>installation</strong>s<br />
A very effective method <strong>of</strong> obtaining a lowresistance<br />
earth connection is to bury a<br />
conductor in the form <strong>of</strong> a closed loop in the<br />
soil at the bottom <strong>of</strong> the excavation for building<br />
foundations.<br />
The resistance R <strong>of</strong> such an electrode (in<br />
homogeneous soil) is given (approximately) in<br />
ohms by: R =<br />
L<br />
2 � where<br />
L L = = length <strong>of</strong> the buried conductor in metres in metres<br />
ρ = soil resistivity in ohm-metres<br />
For n rods: R =<br />
n L<br />
1 �<br />
where<br />
Fig. E20 : Conductor buried below the level <strong>of</strong> the foundations,<br />
i.e. not in the concrete<br />
Earthing schemes<br />
.6 Installation and measurements <strong>of</strong> earth<br />
electrodes<br />
The quality <strong>of</strong> an earth electrode (resistance as low as possible) depends essentially<br />
on two factors:<br />
b Installation method<br />
b Type <strong>of</strong> soil<br />
Installation methods<br />
Three common types <strong>of</strong> <strong>installation</strong> will be discussed:<br />
Buried ring (see Fig. E20)<br />
This solution is strongly recommended, particularly in the case <strong>of</strong> a new building.<br />
The electrode should be buried around the perimeter <strong>of</strong> the excavation made for<br />
the foundations. It is important that the bare conductor be in intimate contact with<br />
the soil (and not placed in the gravel or aggregate hard-core, <strong>of</strong>ten forming a base<br />
for concrete). At least four (widely-spaced) vertically arranged conductors from the<br />
electrode should be provided for the <strong>installation</strong> connections and, where possible,<br />
any reinforcing rods in concrete work should be connected to the electrode.<br />
The conductor forming the earth electrode, particularly when it is laid in an<br />
excavation for foundations, must be in the earth, at least 50 cm below the hard-core<br />
or aggregate base for the concrete foundation. Neither the electrode nor the vertical<br />
rising conductors to the ground floor, should ever be in contact with the foundation<br />
concrete.<br />
For existing buildings, the electrode conductor should be buried around the outside<br />
wall <strong>of</strong> the premises to a depth <strong>of</strong> at least 1 metre. As a general rule, all vertical<br />
connections from an electrode to above-ground level should be insulated for the<br />
nominal LV voltage (600-1,000 V).<br />
The conductors may be:<br />
b Copper: Bare cable (u 25 mm 2 ) or multiple-strip (u 25 mm 2 and u 2 mm thick)<br />
b Aluminium with lead jacket: Cable (u 35 mm 2 )<br />
b Galvanised-steel cable: Bare cable (u 95 mm 2 ) or multiple-strip (u 100 mm 2<br />
and u 3 mm thick)<br />
The approximate resistance R <strong>of</strong> the electrode in ohms:<br />
R =<br />
L<br />
2 � where<br />
L = length where <strong>of</strong> the buried conductor in metres<br />
L = length <strong>of</strong> conductor in metres<br />
ρ = resistivity <strong>of</strong> the soil in ohm-metres (see “Influence <strong>of</strong> the type <strong>of</strong> soil” next page)<br />
Earthing rods (see Fig. E2 )<br />
Vertically driven earthing rods are <strong>of</strong>ten used for existing buildings, and for improving<br />
(i.e. reducing the resistance <strong>of</strong>) existing earth electrodes.<br />
The rods may be:<br />
b Copper or (more commonly) copper-clad steel. The latter are generally 1 or<br />
2 metres long and provided with screwed ends and sockets in order to reach<br />
considerable depths, if necessary (for instance, the water-table level in areas <strong>of</strong> high<br />
soil resistivity)<br />
b Galvanised (see note (1) next page) steel pipe u 25 mm diameter or<br />
rod u 15 mm diameter, u 2 metres long in each case.<br />
Fig. E21 : Earthing rods<br />
L u 3 m<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Rods connected in parallel<br />
E<br />
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E - Distribution in low-voltage <strong>installation</strong>s<br />
For a vertical plate electrode: R =<br />
L<br />
0.8 �<br />
Measurements on earth electrodes in similar<br />
soils are useful to determine the resistivity<br />
value to be applied for the <strong>design</strong> <strong>of</strong> an earthelectrode<br />
system<br />
Fig. E22 : Vertical plate<br />
2 mm thickness (Cu)<br />
(1) Where galvanised conducting materials are used for earth<br />
electrodes, sacrificial cathodic protection anodes may be<br />
necessary to avoid rapid corrosion <strong>of</strong> the electrodes where<br />
the soil is aggressive. Specially prepared magnesium anodes<br />
(in a porous sack filled with a suitable “soil”) are available for<br />
direct connection to the electrodes. In such circumstances, a<br />
specialist should be consulted<br />
It is <strong>of</strong>ten necessary to use more than one rod, in which case the spacing between<br />
them should exceed the depth to which they are driven, by a factor <strong>of</strong> 2 to 3.<br />
The total resistance (in homogeneous soil) is then equal to the resistance <strong>of</strong> one rod,<br />
divided by the number <strong>of</strong> rods in question. The approximate resistance R obtained is:<br />
R =<br />
n L<br />
1 �<br />
if the distance separating the rods > 4L<br />
where<br />
Earthing schemes<br />
where<br />
L = the length <strong>of</strong> the rod in metres<br />
ρ = resistivity <strong>of</strong> the soil in ohm-metres (see “Influence <strong>of</strong> the type <strong>of</strong> soil” below)<br />
n = the number <strong>of</strong> rods<br />
Vertical plates (see Fig. E22)<br />
Rectangular plates, each side <strong>of</strong> which must be u 0.5 metres, are commonly used as<br />
earth electrodes, being buried in a vertical plane such that the centre <strong>of</strong> the plate is<br />
at least 1 metre below the surface <strong>of</strong> the soil.<br />
The plates may be:<br />
b Copper <strong>of</strong> 2 mm thickness<br />
b Galvanised (1) steel <strong>of</strong> 3 mm thickness<br />
The resistance R in ohms is given (approximately), by:<br />
R =<br />
L<br />
0.8 �<br />
L = the perimeter <strong>of</strong> the plate in metres<br />
ρ = resistivity <strong>of</strong> the soil in ohm-metres (see “Influence <strong>of</strong> the type <strong>of</strong> soil” below)<br />
Influence <strong>of</strong> the type <strong>of</strong> soil<br />
Type <strong>of</strong> soil Mean value <strong>of</strong> resistivity<br />
in Ωm<br />
Swampy soil, bogs 1 - 30<br />
Silt alluvium 20 - 100<br />
Humus, leaf mould 10 - 150<br />
Peat, turf 5 - 100<br />
S<strong>of</strong>t clay 50<br />
Marl and compacted clay 100 - 200<br />
Jurassic marl 30 - 40<br />
Clayey sand 50 - 500<br />
Siliceous sand 200 - 300<br />
Stoney ground 1,500 - 3,000<br />
Grass-covered-stoney sub-soil 300 - 500<br />
Chalky soil 100 - 300<br />
Limestone 1,000 - 5,000<br />
Fissured limestone 500 - 1,000<br />
Schist, shale 50 - 300<br />
Mica schist 800<br />
Granite and sandstone 1,500 - 10,000<br />
Modified granite and sandstone 100 - 600<br />
Fig. E23 : Resistivity (Ωm) for different types <strong>of</strong> soil<br />
Type <strong>of</strong> soil Average value <strong>of</strong> resistivity<br />
in Ωm<br />
Fertile soil, compacted damp fill 50<br />
Arid soil, gravel, uncompacted non-uniform fill 500<br />
Stoney soil, bare, dry sand, fissured rocks 3,000<br />
Fig. E24 : Average resistivity (Ωm) values for approximate earth-elect<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
E - Distribution in low-voltage <strong>installation</strong>s<br />
Earthing schemes<br />
Measurement and constancy <strong>of</strong> the resistance between an<br />
earth electrode and the earth<br />
The resistance <strong>of</strong> the electrode/earth interface rarely remains constant<br />
Among the principal factors affecting this resistance are the following:<br />
b Humidity <strong>of</strong> the soil<br />
The seasonal changes in the moisture content <strong>of</strong> the soil can be significant at depths<br />
<strong>of</strong> up to 2 meters.<br />
At a depth <strong>of</strong> 1 metre the resistivity and therefore the resistance can vary by a ratio<br />
<strong>of</strong> 1 to 3 between a wet winter and a dry summer in temperate regions<br />
b Frost<br />
Frozen earth can increase the resistivity <strong>of</strong> the soil by several orders <strong>of</strong> magnitude.<br />
This is one reason for recommending the <strong>installation</strong> <strong>of</strong> deep electrodes, in particular<br />
in cold climates<br />
b Ageing<br />
The materials used for electrodes will generally deteriorate to some extent for<br />
various reasons, for example:<br />
v Chemical reactions (in acidic or alkaline soils)<br />
v Galvanic: due to stray DC currents in the earth, for example from electric railways,<br />
etc. or due to dissimilar metals forming primary cells. Different soils acting on<br />
sections <strong>of</strong> the same conductor can also form cathodic and anodic areas with<br />
consequent loss <strong>of</strong> surface metal from the latter areas. Unfortunately, the most<br />
favourable conditions for low earth-electrode resistance (i.e. low soil resistivity) are<br />
also those in which galvanic currents can most easily flow.<br />
b Oxidation<br />
Brazed and welded joints and connections are the points most sensitive to oxidation.<br />
Thorough cleaning <strong>of</strong> a newly made joint or connection and wrapping with a suitable<br />
greased-tape binding is a commonly used preventive measure.<br />
Measurement <strong>of</strong> the earth-electrode resistance<br />
There must always be one or more removable links to isolate an earth electrode so<br />
that it can be tested.<br />
There must always be removable links which allow the earth electrode to be isolated<br />
from the <strong>installation</strong>, so that periodic tests <strong>of</strong> the earthing resistance can be carried<br />
out. To make such tests, two auxiliary electrodes are required, each consisting <strong>of</strong> a<br />
vertically driven rod.<br />
b Ammeter method (see Fig. E25)<br />
Fig. E25 : Measurement <strong>of</strong> the resistance to earth <strong>of</strong> the earth electrode <strong>of</strong> an <strong>installation</strong> by<br />
means <strong>of</strong> an ammeter<br />
U<br />
A = RT + Rt<br />
= Tt1<br />
1<br />
i1<br />
U<br />
B = Rt + R t t<br />
t = 1 2<br />
1 2<br />
i2<br />
U<br />
C = Rt + R t T<br />
T = 2<br />
2<br />
i3<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
T<br />
A<br />
t2<br />
When the source voltage U is constant (adjusted to be the same value for each test)<br />
then:<br />
U � 1 1 1�<br />
RT<br />
= � + �<br />
2 � i i i<br />
�<br />
1 3 2 �<br />
U<br />
t1<br />
E 3<br />
© Schneider Electric - all rights reserved
E 4<br />
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E - Distribution in low-voltage <strong>installation</strong>s<br />
Earthing schemes<br />
In order to avoid errors due to stray earth currents (galvanic -DC- or leakage currents<br />
from power and communication networks and so on) the test current should be<br />
AC, but at a different frequency to that <strong>of</strong> the power system or any <strong>of</strong> its harmonics.<br />
Instruments using hand-driven generators to make these measurements usually<br />
produce an AC voltage at a frequency <strong>of</strong> between 85 Hz and 135 Hz.<br />
The distances between the electrodes are not critical and may be in different<br />
directions from the electrode being tested, according to site conditions. A number <strong>of</strong><br />
tests at different spacings and directions are generally made to cross-check the test<br />
results.<br />
b Use <strong>of</strong> a direct-reading earthing-resistance ohmmeter<br />
These instruments use a hand-driven or electronic-type AC generator, together<br />
with two auxiliary electrodes, the spacing <strong>of</strong> which must be such that the zone <strong>of</strong><br />
influence <strong>of</strong> the electrode being tested should not overlap that <strong>of</strong> the test electrode (C).<br />
The test electrode (C) furthest from the electrode (X) under test, passes a current<br />
through the earth and the electrode under test, while the second test electrode (P)<br />
picks up a voltage. This voltage, measured between (X) and (P), is due to the test<br />
current and is a measure <strong>of</strong> the contact resistance (<strong>of</strong> the electrode under test) with<br />
earth. It is clear that the distance (X) to (P) must be carefully chosen to give accurate<br />
results. If the distance (X) to (C) is increased, however, the zones <strong>of</strong> resistance <strong>of</strong><br />
electrodes (X) and (C) become more remote, one from the other, and the curve <strong>of</strong><br />
potential (voltage) becomes more nearly horizontal about the point (O).<br />
In practical tests, therefore, the distance (X) to (C) is increased until readings taken<br />
with electrode (P) at three different points, i.e. at (P) and at approximately 5 metres<br />
on either side <strong>of</strong> (P), give similar values. The distance (X) to (P) is generally about<br />
0.68 <strong>of</strong> the distance (X) to (C).<br />
voltage-drop due<br />
to the resistance<br />
<strong>of</strong> electrode (X)<br />
voltage-drop due<br />
to the resistance<br />
<strong>of</strong> electrode (C)<br />
Fig. E26 : Measurement <strong>of</strong> the resistance to the mass <strong>of</strong> earth <strong>of</strong> electrode (X) using an earthelectrode-testing<br />
ohmmeter.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G<br />
V<br />
I<br />
X P<br />
C<br />
a) the principle <strong>of</strong> measurement is based on assumed homogeneous soil conditions. Where the<br />
zones <strong>of</strong> influence <strong>of</strong> electrodes C and X overlap, the location <strong>of</strong> test electrode P is difficult to<br />
determine for satisfactory results.<br />
X P<br />
b) showing the effect on the potential gradient when (X) and (C) are widely spaced. The location<br />
<strong>of</strong> test electrode P is not critical and can be easily determined.<br />
VG<br />
O<br />
O<br />
C<br />
VG
E - Distribution in low-voltage <strong>installation</strong>s<br />
Distribution switchboards, including the main<br />
LV switchboard (MLVS), are critical to the<br />
dependability <strong>of</strong> an <strong>electrical</strong> <strong>installation</strong>.<br />
They must comply with well-defined standards<br />
governing the <strong>design</strong> and construction <strong>of</strong><br />
LV switchgear assemblies<br />
The load requirements dictate the type <strong>of</strong><br />
distribution switchboard to be installed<br />
2 The <strong>installation</strong> system<br />
2.1 Distribution switchboards<br />
A distribution switchboard is the point at which an incoming-power supply divides<br />
into separate circuits, each <strong>of</strong> which is controlled and protected by the fuses or<br />
switchgear <strong>of</strong> the switchboard. A distribution switchboard is divided into a number<br />
<strong>of</strong> functional units, each comprising all the <strong>electrical</strong> and mechanical elements<br />
that contribute to the fulfilment <strong>of</strong> a given function. It represents a key link in the<br />
dependability chain.<br />
Consequently, the type <strong>of</strong> distribution switchboard must be perfectly adapted to its<br />
application. Its <strong>design</strong> and construction must comply with applicable standards and<br />
working practises.<br />
The distribution switchboard enclosure provides dual protection:<br />
b Protection <strong>of</strong> switchgear, indicating instruments, relays, fusegear, etc. against<br />
mechanical impacts, vibrations and other external influences likely to interfere with<br />
operational integrity (EMI, dust, moisture, vermin, etc.)<br />
b The protection <strong>of</strong> human life against the possibility <strong>of</strong> direct and indirect electric<br />
shock (see degree <strong>of</strong> protection IP and the IK index in section 3.3 <strong>of</strong> <strong>Chapter</strong> E).<br />
Types <strong>of</strong> distribution switchboards<br />
Distribution switchboards may differ according to the kind <strong>of</strong> application and the<br />
<strong>design</strong> principle adopted (notably in the arrangement <strong>of</strong> the busbars).<br />
Distribution switchboards according to specific applications<br />
The principal types <strong>of</strong> distribution switchboards are:<br />
b The main LV switchboard - MLVS - (see Fig. E27a)<br />
b Motor control centres - MCC - (see Fig. E27b)<br />
b Sub-distribution switchboards (see Fig. E28)<br />
b Final distribution switchboards (see Fig. E29)<br />
Distribution switchboards for specific applications (e.g. heating, lifts, industrial<br />
processes) can be located:<br />
b Adjacent to the main LV switchboard, or<br />
b Near the application concerned<br />
Sub-distribution and final distribution switchboards are generally distributed<br />
throughout the site.<br />
a b<br />
Fig. E27 : [a] A main LV switchboard - MLVS - (Prisma Plus P) with incoming circuits in the form<br />
<strong>of</strong> busways - [b] A LV motor control centre - MCC - (Okken)<br />
a b c<br />
Fig. E28 : A sub-distribution switchboard (Prisma Plus G) Fig. E29 : Final distribution switchboards [a] Prisma Plus G Pack; [b] Kaedra; [c] mini-Pragma<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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E16<br />
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E - Distribution in low-voltage <strong>installation</strong>s<br />
A distinction is made between:<br />
b Traditional distribution switchboards in which<br />
switchgear and fusegear, etc. are fixed to a<br />
chassis at the rear <strong>of</strong> an enclosure<br />
b Functional distribution switchboards for<br />
specific applications, based on modular and<br />
standardised <strong>design</strong>.<br />
Fig. E30 : Assembly <strong>of</strong> a final distribution switchboard with<br />
fixed functional units (Prisma Plus G)<br />
Fig. E31 : Distribution switchboard with disconnectable<br />
functional units<br />
Fig. E32 : Distribution switchboard with withdrawable functional<br />
units in drawers<br />
2 The <strong>installation</strong> system<br />
Two technologies <strong>of</strong> distribution switchboards<br />
Traditional distribution switchboards<br />
Switchgear and fusegear, etc. are normally located on a chassis at the rear <strong>of</strong> the<br />
enclosure. Indications and control devices (meters, lamps, pushbuttons, etc.) are<br />
mounted on the front face <strong>of</strong> the switchboard.<br />
The placement <strong>of</strong> the components within the enclosure requires very careful study,<br />
taking into account the dimensions <strong>of</strong> each item, the connections to be made to it,<br />
and the clearances necessary to ensure safe and trouble-free operation. .<br />
Functional distribution switchboards<br />
<strong>General</strong>ly dedicated to specific applications, these distribution switchboards are<br />
made up <strong>of</strong> functional modules that include switchgear devices together with<br />
standardised accessories for mounting and connections, ensuring a high level <strong>of</strong><br />
reliability and a great capacity for last-minute and future changes.<br />
b Many advantages<br />
The use <strong>of</strong> functional distribution switchboards has spread to all levels <strong>of</strong> LV<br />
<strong>electrical</strong> distribution, from the main LV switchboard (MLVS) to final distribution<br />
switchboards, due to their many advantages:<br />
v System modularity that makes it possible to integrate numerous functions in a<br />
single distribution switchboard, including protection, control, technical management<br />
and monitoring <strong>of</strong> <strong>electrical</strong> <strong>installation</strong>s. Modular <strong>design</strong> also enhances distribution<br />
switchboard maintenance, operation and upgrades<br />
v Distribution switchboard <strong>design</strong> is fast because it simply involves adding functional<br />
modules<br />
v Prefabricated components can be mounted faster<br />
v Finally, these distribution switchboards are subjected to type tests that ensure a<br />
high degree <strong>of</strong> dependability.<br />
The new Prisma Plus G and P ranges <strong>of</strong> functional distribution switchboards from<br />
Schneider Electric cover needs up to 3200 A and <strong>of</strong>fer:<br />
v Flexibility and ease in building distribution switchboards<br />
v Certification <strong>of</strong> a distribution switchboard complying with standard IEC 60439 and<br />
the assurance <strong>of</strong> servicing under safe conditions<br />
v Time savings at all stages, from <strong>design</strong> to <strong>installation</strong>, operation and modifications<br />
or upgrades<br />
v Easy adaptation, for example to meet the specific work habits and standards in<br />
different countries<br />
Figures E27a, E28 and E29 show examples <strong>of</strong> functional distribution switchboards<br />
ranging for all power ratings and figure E27b shows a high-power industrial functional<br />
distribution switchboard.<br />
b Main types <strong>of</strong> functional units<br />
Three basic technologies are used in functional distribution switchboards.<br />
v Fixed functional units (see Fig. E30)<br />
These units cannot be isolated from the supply so that any intervention for<br />
maintenance, modifications and so on, requires the shutdown <strong>of</strong> the entire<br />
distribution switchboard. Plug-in or withdrawable devices can however be used to<br />
minimise shutdown times and improve the availability <strong>of</strong> the rest <strong>of</strong> the <strong>installation</strong>.<br />
v Disconnectable functional units (see Fig. E31)<br />
Each functional unit is mounted on a removable mounting plate and provided with a<br />
means <strong>of</strong> isolation on the upstream side (busbars) and disconnecting facilities on the<br />
downstream (outgoing circuit) side. The complete unit can therefore be removed for<br />
servicing, without requiring a general shutdown.<br />
v Drawer-type withdrawable functional units (see Fig. E32)<br />
The switchgear and associated accessories for a complete function are mounted on<br />
a drawer-type horizontally withdrawable chassis. The function is generally complex<br />
and <strong>of</strong>ten concerns motor control.<br />
Isolation is possible on both the upstream and downstream sides by the complete<br />
withdrawal <strong>of</strong> the drawer, allowing fast replacement <strong>of</strong> a faulty unit without deenergising<br />
the rest <strong>of</strong> the distribution switchboard.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
E - Distribution in low-voltage <strong>installation</strong>s<br />
Compliance with applicable standards is<br />
essential in order to ensure an adequate<br />
degree <strong>of</strong> dependability<br />
Three elements <strong>of</strong> standard IEC 60439-1<br />
contribute significantly to dependability:<br />
b Clear definition <strong>of</strong> functional units<br />
b Forms <strong>of</strong> separation between adjacent<br />
functional units in accordance with user<br />
requirements<br />
b Clearly defined routine tests and type tests<br />
Standards<br />
Different standards<br />
Certain types <strong>of</strong> distribution switchboards (in particular, functional distribution<br />
switchboards) must comply with specific standards according to the application or<br />
environment involved.<br />
The reference international standard is IEC 60439-1 type-tested and partially typetested<br />
assemblies<br />
Standard IEC 60439-1<br />
b Categories <strong>of</strong> assemblies<br />
Standard IEC 60439-1 distinguishes between two categories <strong>of</strong> assemblies:<br />
v Type-tested LV switchgear and controlgear assemblies (TTA), which do not diverge<br />
significantly from an established type or system for which conformity is ensured by<br />
the type tests provided in the standard<br />
v Partially type-tested LV switchgear and controlgear assemblies (PTTA), which may<br />
contain non-type-tested arrangements provided that the latter are derived from typetested<br />
arrangements<br />
When implemented in compliance with pr<strong>of</strong>essional work standards and<br />
manufacturer instructions by qualified personnel, they <strong>of</strong>fer the same level <strong>of</strong> safety<br />
and quality.<br />
b Functional units<br />
The same standard defines functional units:<br />
v Part <strong>of</strong> an assembly comprising all the <strong>electrical</strong> and mechanical elements that<br />
contribute to the fulfilment <strong>of</strong> the same function<br />
v The distribution switchboard includes an incoming functional unit and one or more<br />
functional units for outgoing circuits, depending on the operating requirements <strong>of</strong> the<br />
<strong>installation</strong><br />
What is more, distribution switchboard technologies use functional units that may be<br />
fixed, disconnectable or withdrawable (see section 3.1 <strong>of</strong> <strong>Chapter</strong> E).<br />
b Forms (see Fig. E33)<br />
Separation <strong>of</strong> functional units within the assembly is provided by forms that are<br />
specified for different types <strong>of</strong> operation.<br />
The various forms are numbered from 1 to 4 with variations labelled “a” or “b”. Each<br />
step up (from 1 to 4) is cumulative, i.e. a form with a higher number includes the<br />
characteristics <strong>of</strong> forms with lower numbers. The standard distinguishes:<br />
v Form 1: No separation<br />
v Form 2: Separation <strong>of</strong> busbars from the functional units<br />
v Form 3: Separation <strong>of</strong> busbars from the functional units and separation <strong>of</strong> all<br />
functional units, one from another, except at their output terminals<br />
v Form 4: As for Form 3, but including separation <strong>of</strong> the outgoing terminals <strong>of</strong> all<br />
functional units, one from another<br />
The decision on which form to implement results from an agreement between the<br />
manufacturer and the user.<br />
The Prima Plus functional range <strong>of</strong>fers solutions for forms 1, 2b, 3b, 4a, 4b.<br />
Form 1 Form 2a Form 2b Form 3a<br />
Form 3b Form 4a Form 4b<br />
Fig. E33 : Representation <strong>of</strong> different forms <strong>of</strong> LV functional distribution switchboards<br />
2 The <strong>installation</strong> system<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Busbar<br />
Separation<br />
E17<br />
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E18<br />
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E - Distribution in low-voltage <strong>installation</strong>s<br />
Total accessibility <strong>of</strong> <strong>electrical</strong> information and<br />
intelligent distribution switchboards are now a<br />
reality<br />
Two types <strong>of</strong> distribution are possible:<br />
b By insulated wires and cables<br />
b By busbar trunking (busways)<br />
2 The <strong>installation</strong> system<br />
b Type tests and routine tests<br />
They ensure compliance <strong>of</strong> each distribution switchboard with the standard. The<br />
availability <strong>of</strong> test documents certified by independent organisations is a guarantee<br />
for users.<br />
Remote monitoring and control <strong>of</strong> the <strong>electrical</strong> <strong>installation</strong><br />
Remote monitoring and control are no longer limited to large <strong>installation</strong>s.<br />
These functions are increasingly used and provide considerable cost savings.<br />
The main potential advantages are:<br />
b Reductions in energy bills<br />
b Reductions in structural costs to maintain the <strong>installation</strong> in running order<br />
b Better use <strong>of</strong> the investment, notably concerning optimisation <strong>of</strong> the <strong>installation</strong> life<br />
cycle<br />
b Greater satisfaction for energy users (in a building or in process industries) due to<br />
improved power availability and/or quality<br />
The above possibilities are all the more an option given the current deregulation <strong>of</strong><br />
the <strong>electrical</strong>-energy sector.<br />
Modbus is increasingly used as the open standard for communication within the<br />
distribution switchboard and between the distribution switchboard and customer<br />
power monitoring and control applications. Modbus exists in two forms, twisted pair<br />
(RS 485) and Ethernet-TCP/IP (IEEE 802.3).<br />
The www.modbus.org site presents all bus specifications and constantly updates the<br />
list <strong>of</strong> products and companies using the open industrial standard.<br />
The use <strong>of</strong> web technologies has largely contributed to wider use by drastically<br />
reducing the cost <strong>of</strong> accessing these functions through the use <strong>of</strong> an interface that is<br />
now universal (web pages) and a degree <strong>of</strong> openness and upgradeability that simply<br />
did not exist just a few years ago.<br />
2.2 Cables and busway trunking<br />
Distribution by insulated conductors and cables<br />
Definitions<br />
b Conductor<br />
A conductor comprises a single metallic core with or without an insulating envelope.<br />
b Cable<br />
A cable is made up <strong>of</strong> a number <strong>of</strong> conductors, <strong>electrical</strong>ly separated, but joined<br />
mechanically, generally enclosed in a protective flexible sheath.<br />
b Cableway<br />
The term cableway refers to conductors and/or cables together with the means <strong>of</strong><br />
support and protection, etc. for example : cable trays, ladders, ducts, trenches, and<br />
so on… are all “cableways”.<br />
Conductor marking<br />
Conductor identification must always respect the following three <strong>rules</strong>:<br />
b Rule 1<br />
The double colour green and yellow is strictly reserved for the PE and PEN<br />
protection conductors.<br />
b Rule 2<br />
v When a circuit comprises a neutral conductor, it must be light blue or marked “1” for<br />
cables with more than five conductors<br />
v When a circuit does not have a neutral conductor, the light blue conductor may be<br />
used as a phase conductor if it is part <strong>of</strong> a cable with more than one conductor<br />
b Rule 3<br />
Phase conductors may be any colour except:<br />
v Green and yellow<br />
v Green<br />
v Yellow<br />
v Light blue (see rule 2)<br />
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Conductors in a cable are identified either by their colour or by numbers (see Fig. E34).<br />
Number <strong>of</strong> Circuit Fixed cableways<br />
conductors Insulated conductors Rigid and flexible multi-<br />
in circuit conductor cables<br />
Ph Ph Pn N PE Ph Ph Ph N PE<br />
1 Protection or earth G/Y<br />
2 Single-phase between phases b b BL LB<br />
Single-phase between phase and neutral b LB BL LB<br />
Single-phase between phase and neutral b G/Y BL G/Y<br />
+ protection conductor<br />
3 Three-phase without neutral b b b BL B LB<br />
2 phases + neutral b b LB BL B LB<br />
2 phases + protection conductor b b G/Y BL LB G/Y<br />
Single-phase between phase and neutral b LB G/Y BL LB G/Y<br />
+ protection conductor<br />
4 Three-phase with neutral b b b LB BL B BL LB<br />
Three-phase with neutral + protection conductor b b b G/Y BL B LB G/Y<br />
2 phases + neutral + protection conductor b b LB G/Y BL B LB G/Y<br />
Three-phase with PEN conductor b b b G/Y BL B LB G/Y<br />
5 Three-phase + neutral + protection conductor b b b LB G/Y BL B BL LB G/Y<br />
> 5 Protection conductor: G/Y - Other conductors: BL: with numbering<br />
The number “1” is reserved for the neutral conductor if it exists<br />
G/Y: Green and yellow BL: Black b : As indicated in rule 3 LB: Light blue B: Brown<br />
Fig. E34 : Conductor identification according to the type <strong>of</strong> circuit<br />
N<br />
Black conductor<br />
Light blue conductor<br />
Fig. E35 : Conductor identification on a circuit-breaker with a<br />
phase and a neutral<br />
2 The <strong>installation</strong> system<br />
Note: If the circuit includes a protection conductor and if the available cable does not<br />
have a green and yellow conductor, the protection conductor may be:<br />
b A separate green and yellow conductor<br />
b The blue conductor if the circuit does not have a neutral conductor<br />
b A black conductor if the circuit has a neutral conductor<br />
In the last two cases, the conductor used must be marked by green and yellow<br />
bands or markings at the ends and on all visible lengths <strong>of</strong> the conductor.<br />
Equipment power cords are marked similar to multi-conductor cables (see Fig. E35).<br />
Distribution and <strong>installation</strong> methods (see Fig. E36)<br />
Distribution takes place via cableways that carry single insulated conductors or<br />
cables and include a fixing system and mechanical protection.<br />
Floor subdistribution<br />
swichboard<br />
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Main LV switchboard<br />
(MLVS)<br />
Fig. E36 : Radial distribution using cables in a hotel<br />
Heating, etc.<br />
Final<br />
distribution<br />
swichboard<br />
Building utilities sub-distribution swichboard<br />
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Busways, also referred to as busbar trunking<br />
systems, stand out for their ease <strong>of</strong> <strong>installation</strong>,<br />
flexibility and number <strong>of</strong> possible connection<br />
points<br />
2 The <strong>installation</strong> system<br />
Busbar trunking (busways)<br />
Busbar trunking is intended to distribute power (from 20 A to 5000 A) and lighting<br />
(in this application, the busbar trunking may play a dual role <strong>of</strong> supplying <strong>electrical</strong><br />
power and physically holding the lights).<br />
Busbar trunking system components<br />
A busbar trunking system comprises a set <strong>of</strong> conductors protected by an enclosure<br />
(see Fig. E37). Used for the transmission and distribution <strong>of</strong> <strong>electrical</strong> power, busbar<br />
trunking systems have all the necessary features for fitting: connectors, straights,<br />
angles, fixings, etc. The tap-<strong>of</strong>f points placed at regular intervals make power<br />
available at every point in the <strong>installation</strong>.<br />
Straight trunking Tap-<strong>of</strong>f points to<br />
distribute current<br />
Power Unit Range <strong>of</strong> clip-on tap-<strong>of</strong>f units to<br />
connect a load (e.g.: a machine) to<br />
the busbar trunking<br />
Fig. E37 : Busbar trunking system <strong>design</strong> for distribution <strong>of</strong> currents from 25 to 4000 A.<br />
The various types <strong>of</strong> busbar trunking:<br />
Busbar trunking systems are present at every level in <strong>electrical</strong> distribution: from<br />
the link between the transformer and the low voltage switch switchboard (MLVS)<br />
to the distribution <strong>of</strong> power sockets and lighting to <strong>of</strong>fices, or power distribution to<br />
workshops.<br />
Fig. E38 : Radial distribution using busways<br />
We talk about a distributed network architecture.<br />
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Fixing system for ceilings, walls or<br />
raised floor, etc.<br />
Angle<br />
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2 The <strong>installation</strong> system<br />
There are essentially three categories <strong>of</strong> busways.<br />
b Transformer to MLVS busbar trunking<br />
Installation <strong>of</strong> the busway may be considered as permanent and will most likely never<br />
be modified. There are no tap-<strong>of</strong>f points.<br />
Frequently used for short runs, it is almost always used for ratings above 1,600 /<br />
2,000 A, i.e. when the use <strong>of</strong> parallel cables makes <strong>installation</strong> impossible. Busways<br />
are also used between the MLVS and downstream distribution switchboards.<br />
The characteristics <strong>of</strong> main-distribution busways authorize operational currents from<br />
1,000 to 5,000 A and short-circuit withstands up to 150 kA.<br />
b Sub-distribution busbar trunking with low or high tap-<strong>of</strong>f densities<br />
Downstream <strong>of</strong> main-distribution busbar trunking , two types <strong>of</strong> applications must be<br />
supplied:<br />
v Mid-sized premises (industrial workshops with injection presses and metalwork<br />
machines or large supermarkets with heavy loads). The short-circuit and current<br />
levels can be fairly high (respectively 20 to 70 kA and 100 to 1,000 A)<br />
v Small sites (workshops with machine-tools, textile factories with small machines,<br />
supermarkets with small loads). The short-circuit and current levels are lower<br />
(respectively 10 to 40 kA and 40 to 400 A)<br />
Sub-distribution using busbar trunking meets user needs in terms <strong>of</strong>:<br />
v Modifications and upgrades given the high number <strong>of</strong> tap-<strong>of</strong>f points<br />
v Dependability and continuity <strong>of</strong> service because tap-<strong>of</strong>f units can be connected<br />
under energized conditions in complete safety<br />
The sub-distribution concept is also valid for vertical distribution in the form <strong>of</strong> 100 to<br />
5,000 A risers in tall buildings.<br />
b Lighting distribution busbar trunking<br />
Lighting circuits can be distributed using two types <strong>of</strong> busbar trunking according to<br />
whether the lighting fixtures are suspended from the busbar trunking or not.<br />
v busbar trunking <strong>design</strong>ed for the suspension <strong>of</strong> lighting fixtures<br />
These busways supply and support light fixtures (industrial reflectors, discharge<br />
lamps, etc.). They are used in industrial buildings, supermarkets, department stores<br />
and warehouses. The busbar trunkings are very rigid and are <strong>design</strong>ed for one or<br />
two 25 A or 40 A circuits. They have tap-<strong>of</strong>f outlets every 0.5 to 1 m.<br />
v busbar trunking not <strong>design</strong>ed for the suspension <strong>of</strong> lighting fixtures<br />
Similar to prefabricated cable systems, these busways are used to supply all types<br />
<strong>of</strong> lighting fixtures secured to the building structure. They are used in commercial<br />
buildings (<strong>of</strong>fices, shops, restaurants, hotels, etc.), especially in false ceilings. The<br />
busbar trunking is flexible and <strong>design</strong>ed for one 20 A circuit. It has tap-<strong>of</strong>f outlets<br />
every 1.2 m to 3 m.<br />
Busbar trunking systems are suited to the requirements <strong>of</strong> a large number <strong>of</strong><br />
buildings.<br />
b Industrial buildings: garages, workshops, farm buildings, logistic centers, etc.<br />
b Commercial areas: stores, shopping malls, supermarkets, hotels, etc.<br />
b Tertiary buildings: <strong>of</strong>fices, schools, hospitals, sports rooms, cruise liners, etc.<br />
Standards<br />
Busbar trunking systems must meet all <strong>rules</strong> stated in IEC 439-2.<br />
This defines the manufacturing arrangements to be complied with in the <strong>design</strong><br />
<strong>of</strong> busbar trunking systems (e.g.: temperature rise characteristics, short-circuit<br />
withstand, mechanical strength, etc.) as well as test methods to check them.<br />
Standard IEC 439-2 defines 13 compulsory type-tests on configurations or system<br />
components..<br />
By assembling the system components on the site according to the assembly<br />
instructions, the contractor benefits from conformity with the standard.<br />
The advantages <strong>of</strong> busbar trunking systems<br />
Flexibility<br />
b Easy to change configuration (on-site modification to change production line<br />
configuration or extend production areas).<br />
b Reusing components (components are kept intact): when an <strong>installation</strong> is subject<br />
to major modifications, the busbar trunking is easy to dismantle and reuse.<br />
b Power availability throughout the <strong>installation</strong> (possibility <strong>of</strong> having a tap-<strong>of</strong>f point<br />
every meter).<br />
b Wide choice <strong>of</strong> tap-<strong>of</strong>f units.<br />
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Simplicity<br />
b Design can be carried out independently from the distribution and layout <strong>of</strong> current<br />
consumers.<br />
b Performances are independent <strong>of</strong> implementation: the use <strong>of</strong> cables requires a lot<br />
<strong>of</strong> derating coefficients.<br />
b Clear distribution layout<br />
b Reduction <strong>of</strong> fitting time: the trunking system allows fitting times to be reduced by<br />
up to 50% compared with a traditional cable <strong>installation</strong>.<br />
b Manufacturer’s guarantee.<br />
b Controlled execution times: the trunking system concept guarantees that there are<br />
no unexpected surprises when fitting. The fitting time is clearly known in advance<br />
and a quick solution can be provided to any problems on site with this adaptable and<br />
scalable equipment.<br />
b Easy to implement: modular components that are easy to handle, simple and quick<br />
to connect.<br />
Dependability<br />
b Reliability guaranteed by being factory-built<br />
b Fool-pro<strong>of</strong> units<br />
b Sequential assembly <strong>of</strong> straight components and tap-<strong>of</strong>f units making it impossible<br />
to make any mistakes<br />
Continuity <strong>of</strong> service<br />
b The large number <strong>of</strong> tap-<strong>of</strong>f points makes it easy to supply power to any new<br />
current consumer. Connecting and disconnecting is quick and can be carried out in<br />
complete safety even when energized. These two operations (adding or modifying)<br />
take place without having to stop operations.<br />
b Quick and easy fault location since current consumers are near to the line<br />
b Maintenance is non existent or greatly reduced<br />
Major contribution to sustainable development<br />
b Busbar trunking systems allow circuits to be combined. Compared with a<br />
traditional cable distribution system, consumption <strong>of</strong> copper raw materials and<br />
insulators is divided by 3 due to the busbar trunking distributed network concept<br />
(see Fig. E39).<br />
Example:<br />
30 m <strong>of</strong> Canalis KS 250A equipped with 10 25 A, four-pole feeders<br />
Distribution type Conductors<br />
Insulators Consumption<br />
Branched<br />
ΣI x k s<br />
ks: clustering coefficient= 0.6<br />
Centralized<br />
ΣI x k s<br />
ks: clustering coefficient= 0.6<br />
I 1 I 2 I 3 I 4 I 5 I 6 I 7<br />
R R R R R R R<br />
I 1 I 2 I 3 I 4 I 5 I 6 I 7<br />
R R R R R R R<br />
Fig. E39 : Example: 30 m <strong>of</strong> Canalis KS 250A equipped with 10 25 A, four-pole feeders<br />
2 The <strong>installation</strong> system<br />
Alu: 128 mm²<br />
Copper equivalent: 86 mm²<br />
Copper: 250 mm²<br />
b Reusable device and all <strong>of</strong> its components are fully recyclable.<br />
b Does not contain PVC and does not generate toxic gases or waste.<br />
b Reduction <strong>of</strong> risks due to exposure to electromagnetic fields.<br />
New functional features for Canalis<br />
Busbar trunking systems are getting even better. Among the new features we can<br />
mention:<br />
b Increased performance with a IP55 protection index and new ratings <strong>of</strong> 160 A<br />
through to 1000 A (Ks).<br />
b New lighting <strong>of</strong>fers with pre-cabled lights and new light ducts.<br />
b New fixing accessories. Quick fixing system, cable ducts, shared support with<br />
“VDI” (voice, data, images) circuits.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
4 kg<br />
12 kg<br />
1 000 Joules<br />
1 600 Joules
E - Distribution in low-voltage <strong>installation</strong>s<br />
2 The <strong>installation</strong> system<br />
Busbar trunking systems are perfectly integrated with the environment:<br />
b white color to enhance the working environment, naturally integrated in a range <strong>of</strong><br />
<strong>electrical</strong> distribution products.<br />
b conformity with European regulations on reducing hazardous materials (RoHS).<br />
Examples <strong>of</strong> Canalis busbar trunking systems<br />
Fig. E40 : Flexible busbar trunking not capable <strong>of</strong> supporting light fittings : Canalis KDP (20 A)<br />
Fig. E41 : Rigid busbar trunking able to support light fittings : Canalis KBA or KBB (25 and 40 A)<br />
Fig. E42 : Lighting duct : Canalis KBX (25 A)<br />
Fig. E43 : A busway for medium power distribution : Canalis KN (40 up to 160 A)<br />
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2 The <strong>installation</strong> system<br />
Fig. E44 : A busway for medium power distribution : Canalis KS (100 up to 1000 A)<br />
Fig. E45 : A busway for high power distribution : Canalis KT (800 up to 1000 A)<br />
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E - Distribution in low-voltage <strong>installation</strong>s<br />
External influences shall be taken into account<br />
when choosing:<br />
b The appropriate measures to ensure the<br />
safety <strong>of</strong> persons (in particular in special<br />
locations or <strong>electrical</strong> <strong>installation</strong>s)<br />
b The characteristics <strong>of</strong> <strong>electrical</strong> equipment,<br />
such as degree <strong>of</strong> protection (IP), mechanical<br />
withstand (IK), etc.<br />
If several external influences appear at the<br />
same time, they can have independent or<br />
mutual effects and the degree <strong>of</strong> protection must<br />
be chosen accordingly<br />
3 External influences<br />
(IEC 60364-5-51)<br />
3.1 Definition and reference standards<br />
Every <strong>electrical</strong> <strong>installation</strong> occupies an environment that presents a variable degree<br />
<strong>of</strong> risk:<br />
b For people<br />
b For the equipment constituting the <strong>installation</strong><br />
Consequently, environmental conditions influence the definition and choice <strong>of</strong><br />
appropriate <strong>installation</strong> equipment and the choice <strong>of</strong> protective measures for the<br />
safety <strong>of</strong> persons.<br />
The environmental conditions are referred to collectively as “external influences”.<br />
Many national standards concerned with external influences include a classification<br />
scheme which is based on, or which closely resembles, that <strong>of</strong> international standard<br />
IEC 60364-5-51.<br />
3.2 Classification<br />
Each condition <strong>of</strong> external influence is <strong>design</strong>ated by a code comprising a group <strong>of</strong><br />
two capital letters and a number as follows:<br />
First letter (A, B or C)<br />
The first letter relates to the general category <strong>of</strong> external influence :<br />
b A = environment<br />
b B = utilisation<br />
b C = construction <strong>of</strong> buildings<br />
Second letter<br />
The second letter relates to the nature <strong>of</strong> the external influence.<br />
Number<br />
The number relates to the class within each external influence.<br />
Additional letter (optional)<br />
Used only if the effective protection <strong>of</strong> persons is greater than that indicated by the<br />
first IP digit.<br />
When only the protection <strong>of</strong> persons is to be specified, the two digits <strong>of</strong> the IP code<br />
are replaced by the X’s.<br />
Example: IP XXB.<br />
Example<br />
For example the code AC2 signifies:<br />
A = environment<br />
AC = environment-altitude<br />
AC2 = environment-altitude > 2,000 m<br />
3.3 List <strong>of</strong> external influences<br />
Figure E46 below is from IEC 60364-5-51, which should be referred to if further<br />
details are required.<br />
Code External influences Characteristics required for equipment<br />
A - Environment<br />
AA Ambient temperature (°C)<br />
Low High Specially <strong>design</strong>ed equipment or appropriate arrangements<br />
AA1 - 60 °C + 5 °C<br />
AA2 - 40 °C + 5 °C<br />
AA3 - 25 °C + 5 °C<br />
AA4 - 5° C + 40 °C Normal (special precautions in certain cases)<br />
AA5 + 5 °C + 40 °C Normal<br />
AA6 + 5 °C + 60 °C Specially <strong>design</strong>ed equipment or appropriate arrangements<br />
AA7 - 25 °C + 55 °C<br />
AA8 - 50 °C + 40 °C<br />
Fig. E46 : List <strong>of</strong> external influences (taken from Appendix A <strong>of</strong> IEC 60364-5-51) (continued on next page)<br />
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3 External influences<br />
(IEC 60364-5-51)<br />
Code External influences Characteristics required for equipment<br />
A - Environment<br />
AB Atmospheric humidity<br />
Air temperature °C Relative humidity % Absolute humidity g/m 3<br />
Low High Low High Low High<br />
AB1 - 60 °C + 5 °C 3 100 0.003 7 Appropriate arrangements shall be made<br />
AB2 - 40 °C + 5 °C 10 100 0.1 7<br />
AB3 - 25 °C + 5 °C 10 100 0.5 7<br />
AB4 - 5° C + 40 °C 5 95 1 29 Normal<br />
AB5 + 5 °C + 40 °C 5 85 1 25 Normal<br />
AB6 + 5 °C + 60 °C 10 100 1 35 Appropriate arrangements shall be made<br />
AB7 - 25 °C + 55 °C 10 100 0.5 29<br />
AB8 - 50 °C + 40 °C 15 100 0.04 36<br />
AC Altitude<br />
AC1 y 2000 m Normal<br />
AC2 > 2000 m May necessitate precaution (derating factors)<br />
AD Presence <strong>of</strong> water<br />
AD1 Negligible Outdoor or non-weather protected locations IPX0<br />
AD2 Free-falling drops IPX1 or IPX2<br />
AD3 Sprays IPX3<br />
AD4 Splashes IPX4<br />
AD5 Jets Locations where hose water is used regularly IPX5<br />
AD6 Waves Seashore locations (piers, beaches, quays…) IPX6<br />
AD7 Immersion Water 150 mm above the highest point and IPX7<br />
equipment not more than 1m below the surface<br />
AD8 Submersion Equipment is permanently and totally covered IPX8<br />
AE Presence <strong>of</strong> foreign solid bodies<br />
Smallest dimension Example<br />
AE1 Negligible IP0X<br />
AE2 Small objects 2.5 mm Tools IP3X<br />
AE3 Very small objects 1 mm Wire IP4X<br />
AE4 Light dust IP5X if dust penetration is not harmful to functioning<br />
AE5 Moderate dust IP6X if dust should not penetrate<br />
AE6 Heavy dust IP6X<br />
AF Presence <strong>of</strong> corrosive or polluting substances<br />
AF1 Negligible Normal<br />
AF2 Atmospheric According to the nature <strong>of</strong> the substance<br />
AF3 Intermittent, accidental Protection against corrosion<br />
AF4 Continuous Equipment specially <strong>design</strong>ed<br />
AG Mechanical stress impact<br />
AG1 Low severity Normal<br />
AG2 Medium severity Standard where applicable or reinforced material<br />
AG3 High severity Reinforced protection<br />
AH Vibrations<br />
AH1 Low severity Household or similar Normal<br />
AH2 Medium severity Usual industrial conditions Specially <strong>design</strong>ed equipment or special arrangements<br />
AH3 High severity Severe industrial conditions<br />
AJ Other mechanical stresses<br />
AK Presence <strong>of</strong> flora and/or mould growth<br />
AH1 No hazard Normal<br />
AH2 Hazard<br />
AL Presence <strong>of</strong> fauna<br />
AH1 No hazard Normal<br />
AH2 Hazard<br />
AM Electromagnetic, electrostatic or ionising influences / Low frequency electromagnetic phenomena / Harmonics<br />
AM1 Harmonics, interharmonics Refer to applicable IEC standards<br />
AM2 Signalling voltage<br />
AM3 Voltage amplitude variations<br />
AM4 Voltage unbalance<br />
AM5 Power frequency variations<br />
AM6 Induced low-frequency voltages<br />
AM7 Direct current in a.c. networks<br />
AM8 Radiated magnetic fields<br />
AM9 Electric field<br />
AM21 Induced oscillatory voltages or currents<br />
Fig. E46 : List <strong>of</strong> external influences (taken from Appendix A <strong>of</strong> IEC 60364-5-51) (continued on next page)<br />
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Fig. E46 : List <strong>of</strong> external influences (taken from Appendix A <strong>of</strong> IEC 60364-5-51) (concluded)<br />
3 External influences<br />
(IEC 60364-5-51)<br />
Code External influences Characteristics required for equipment<br />
A - Environment<br />
AM22 Conducted unidirectional transients <strong>of</strong> the nanosecond time scale Refer to applicable IEC standards<br />
AM23 Conducted unidirectional transients <strong>of</strong> the microsecond to the millisecond<br />
time scale<br />
AM24 Conducted oscillatory transients<br />
AM25 Radiated high frequency phenomena<br />
AM31 Electrostatic discharges<br />
AM41 Ionisation<br />
AN Solar radiation<br />
AN1 Low Normal<br />
AN2 Medium<br />
AN3 High<br />
AP Seismic effect<br />
AP1 Negligible Normal<br />
AP2 Low severity<br />
AP3 Medium severity<br />
AP4 High severity<br />
AQ Lightning<br />
AQ1 Negligible Normal<br />
AQ2 Indirect exposure<br />
AQ3 Direct exposure<br />
AR Movement <strong>of</strong> air<br />
AQ1 Low Normal<br />
AQ2 Medium<br />
AQ3 High<br />
AS Wind<br />
AQ1 Low Normal<br />
AQ2 Medium<br />
AQ3 High<br />
B - Utilization<br />
BA Capability <strong>of</strong> persons<br />
BA1 Ordinary Normal<br />
BA2 Children<br />
BA3 Handicapped<br />
BA4 Instructed<br />
BA5 Skilled<br />
BB Electrical resistance <strong>of</strong> human body<br />
BC Contact <strong>of</strong> persons with earth potential<br />
BC1 None Class <strong>of</strong> equipment according to IEC61140<br />
BC2 Low<br />
BC3 Frequent<br />
BC4 Continuous<br />
BD Condition <strong>of</strong> evacuation in case <strong>of</strong> emergency<br />
BD1 Low density / easy exit Normal<br />
BD2 Low density / difficult exit<br />
BD3 High density / easy exit<br />
BD4 High density / difficult exit<br />
BE Nature <strong>of</strong> processed or stored materials<br />
BE1 No significant risks Normal<br />
BE2 Fire risks<br />
BE3 Explosion risks<br />
BE4 Contamination risks<br />
C - Construction <strong>of</strong> building<br />
CA Construction materials<br />
CA1 Non combustible Normal<br />
CA2 Combustible<br />
CB Building <strong>design</strong><br />
CB1 Negligible risks Normal<br />
CB2 Propagation <strong>of</strong> fire<br />
CB3 Movement<br />
CB4 lexible or unstable<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
E27<br />
© Schneider Electric - all rights reserved
E28<br />
© Schneider Electric - all rights reserved<br />
E - Distribution in low-voltage <strong>installation</strong>s<br />
Code letters<br />
(International Protection)<br />
First characteristic numeral<br />
(numerals 0 to 6, or letter X)<br />
Second characteristic numeral<br />
(numerals 0 to 6, or letter X)<br />
Additional letter (optional)<br />
(letters A, B, C, D)<br />
Supplementary letter (optional)<br />
(letters H, M, S, W)<br />
Fig. E47 : IP Code arrangement<br />
IP 2 3 C H<br />
Where a characteristic numeral is not required to be specified,<br />
it shall be replaced by the letter "X" ("XX" if both numerals<br />
are omitted). Additional letters and/or supplementary letters<br />
may be omitted without replacement.<br />
3 External influences<br />
(IEC 60364-5-51)<br />
3.4 Protection provided for enclosed equipment:<br />
codes IP and IK<br />
IP code definition (see Fig. E47)<br />
The degree <strong>of</strong> protection provided by an enclosure is indicated in the IP code,<br />
recommended in IEC 60529.<br />
Protection is afforded against the following external influences:<br />
b Penetration by solid bodies<br />
b Protection <strong>of</strong> persons against access to live parts<br />
b Protection against the ingress <strong>of</strong> dust<br />
b Protection against the ingress <strong>of</strong> liquids<br />
Note: the IP code applies to <strong>electrical</strong> equipment for voltages up to and including<br />
72.5 kV.<br />
Elements <strong>of</strong> the IP Code and their meanings<br />
A brief description <strong>of</strong> the IP Code elements is given in the following chart<br />
(see Fig. E48).<br />
Element Numerals<br />
or letters<br />
Code letters<br />
First<br />
characteristic<br />
numeral<br />
Second<br />
characteristic<br />
numeral<br />
0<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
IP<br />
Additional<br />
letter<br />
(optional) A<br />
B<br />
C<br />
D<br />
Supplementary<br />
letter<br />
(optional)<br />
0<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
H<br />
M<br />
S<br />
W<br />
Fig. E48 : Elements <strong>of</strong> the IP Code<br />
Meaning for the protection<br />
<strong>of</strong> equipment<br />
Against ingress <strong>of</strong> solid foreign<br />
objects<br />
(non-protected)<br />
u 50 mm diameter<br />
u 12.5 mm diameter<br />
u 2.5 mm diameter<br />
u 1.0 mm diameter<br />
Dust-protected<br />
Dust-tight<br />
Against ingress <strong>of</strong> water with<br />
harmful effects<br />
(non-protected)<br />
Vertically dripping<br />
Dripping (15° tilted)<br />
Spraying<br />
Splashing<br />
Jetting<br />
Powerful jetting<br />
Temporary immersion<br />
Continuous immersion<br />
Supplementary information specific to:<br />
High-voltage apparatus<br />
Motion during water test<br />
Stationary during water test<br />
Weather conditions<br />
Meaning for the<br />
protection <strong>of</strong> persons<br />
Against access to<br />
hazardous parts with<br />
(non-protected)<br />
Back <strong>of</strong> hand<br />
Finger<br />
Tool<br />
Wire<br />
Wire<br />
Wire<br />
Against access to<br />
hazardous parts with<br />
back <strong>of</strong> hand<br />
Finger<br />
Tool<br />
Wire
E - Distribution in low-voltage <strong>installation</strong>s<br />
3 External influences<br />
(IEC 60364-5-51)<br />
IK Code definition<br />
Standard IEC 62262 defines an IK code that characterises the aptitude <strong>of</strong> equipment<br />
to resist mechanical impacts on all sides (see Fig. E49).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
IK code Impact energy AG code<br />
(in Joules)<br />
00 0<br />
01 y 0.14<br />
02 y 0.20 AG1<br />
03 y 0.35<br />
04 y 0.50<br />
05 y 0.70<br />
06 y 1<br />
07 y 2 AG2<br />
08 y 5 AG3<br />
09 y 10<br />
10 y 20 AG4<br />
Fig. E49 : Elements <strong>of</strong> the IK Code<br />
IP and IK code specifications for distribution switchboards<br />
The degrees <strong>of</strong> protection IP and IK <strong>of</strong> an enclosure must be specified as a function<br />
<strong>of</strong> the different external influences defined by standard IEC 60364-5-51, in particular:<br />
b Presence <strong>of</strong> solid bodies (code AE)<br />
b Presence <strong>of</strong> water (code AD)<br />
b Mechanical stresses (no code)<br />
b Capability <strong>of</strong> persons (code BA)<br />
b …<br />
Prisma Plus switchboards are <strong>design</strong>ed for indoor <strong>installation</strong>.<br />
Unless the <strong>rules</strong>, standards and regulations <strong>of</strong> a specific country stipulate otherwise,<br />
Schneider Electric recommends the following IP and IK values (see Fig. E50 and<br />
Fig. E51 )<br />
IP recommendations<br />
IP codes according to conditions<br />
Normal without risk <strong>of</strong> vertically falling water Technical rooms 30<br />
Normal with risk <strong>of</strong> vertically falling water Hallways 31<br />
Very severe with risk <strong>of</strong> splashing water Workshops 54/55<br />
from all directions<br />
Fig. E50 : IP recommendations<br />
IK recommendations<br />
IK codes according to conditions<br />
No risk <strong>of</strong> major impact Technical rooms 07<br />
Significant risk <strong>of</strong> major impact that could Hallways 08 (enclosure<br />
damage devices with door)<br />
Maximum risk <strong>of</strong> impact that could damage Workshops 10<br />
the enclosure<br />
Fig. E51 : IK recommendations<br />
E29<br />
© Schneider Electric - all rights reserved
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
<strong>Chapter</strong> F<br />
Protection against electric shocks<br />
Contents<br />
<strong>General</strong> F2<br />
1.1 Electric shock F2<br />
1.2 Protection against electric shock F3<br />
1.3 Direct and indirect contact F3<br />
Protection against direct contact F4<br />
2.1 Measures <strong>of</strong> protection against direct contact F4<br />
2.2 Additional measure <strong>of</strong> protection against direct contact F6<br />
Protection against indirect contact F6<br />
3.1 Measures <strong>of</strong> protection: two levels F6<br />
3.2 Automatic disconnection for TT system F7<br />
3.3 Automatic disconnection for TN systems F8<br />
3.4 Automatic disconnection on a second fault in an IT system F10<br />
3.5 Measures <strong>of</strong> protection against direct or indirect contact<br />
without automatic disconnection <strong>of</strong> supply F13<br />
Protection <strong>of</strong> goods in case <strong>of</strong> insulation fault F 7<br />
4.1 Measures <strong>of</strong> protection against fire risk with RCDs F17<br />
4.2 Ground Fault Protection (GFP) F17<br />
Implementation <strong>of</strong> the TT system F 9<br />
5.1 Protective measures F19<br />
5.2 Coordination <strong>of</strong> residual current protective devices F20<br />
Implementation <strong>of</strong> the TN system F23<br />
6.1 Preliminary conditions F23<br />
6.2 Protection against indirect contact F23<br />
6.3 High-sensitivity RCDs F27<br />
6.4 Protection in high fire-risk locations F28<br />
6.5 When the fault current-loop impedance is particularly high F28<br />
Implementation <strong>of</strong> the IT system F29<br />
7.1 Preliminary conditions F29<br />
7.2 Protection against indirect contact F30<br />
7.3 High-sensitivity RCDs F34<br />
7.4 Protection in high fire-risk locations F35<br />
7.5 When the fault current-loop impedance is particularly high F35<br />
Residual current differential devices (RCDs) F36<br />
8.1 Types <strong>of</strong> RCDs F36<br />
8.2 Description F36<br />
8.3 Sensitivity <strong>of</strong> RDCs to disturbances F39<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
F<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
F2<br />
F - Protection against electric shock<br />
When a current exceeding 30 mA passes<br />
through a part <strong>of</strong> a human body, the person<br />
concerned is in serious danger if the current is<br />
not interrupted in a very short time.<br />
The protection <strong>of</strong> persons against electric<br />
shock in LV <strong>installation</strong>s must be provided in<br />
conformity with appropriate national standards<br />
statutory regulations, codes <strong>of</strong> practice, <strong>of</strong>ficial<br />
guides and circulars etc.<br />
Relevant IEC standards include: IEC 60364,<br />
IEC 60479 series, IEC 61008, IEC 61009 and<br />
IEC 60947-2.<br />
10,000<br />
5,000<br />
2,000<br />
1,000<br />
500<br />
200<br />
100<br />
50<br />
20<br />
10<br />
Duration <strong>of</strong> current<br />
flow I (ms)<br />
AC-1<br />
A B<br />
<strong>General</strong><br />
. Electric shock<br />
Fig. F1 : Zones time/current <strong>of</strong> effects <strong>of</strong> AC current on human body when passing from left hand to feet<br />
An electric shock is the pathophysiological effect <strong>of</strong> an electric current through the<br />
human body.<br />
Its passage affects essentially the muscular, circulatory and respiratory functions and<br />
sometimes results in serious burns. The degree <strong>of</strong> danger for the victim is a function<br />
<strong>of</strong> the magnitude <strong>of</strong> the current, the parts <strong>of</strong> the body through which the current<br />
passes, and the duration <strong>of</strong> current flow.<br />
IEC publication 60479-1 updated in 2005 defines four zones <strong>of</strong> current-magnitude/<br />
time-duration, in each <strong>of</strong> which the pathophysiological effects are described (see Fig<br />
F ). Any person coming into contact with live metal risks an electric shock.<br />
Curve C1 shows that when a current greater than 30 mA passes through a human<br />
being from one hand to feet, the person concerned is likely to be killed, unless the<br />
current is interrupted in a relatively short time.<br />
The point 500 ms/100 mA close to the curve C1 corresponds to a probability <strong>of</strong> heart<br />
fibrillation <strong>of</strong> the order <strong>of</strong> 0.14%.<br />
The protection <strong>of</strong> persons against electric shock in LV <strong>installation</strong>s must be provided<br />
in conformity with appropriate national standards and statutory regulations, codes <strong>of</strong><br />
practice, <strong>of</strong>ficial guides and circulars, etc. Relevant IEC standards include: IEC 60364<br />
series, IEC 60479 series, IEC 60755, IEC 61008 series, IEC 61009 series and IEC<br />
60947-2.<br />
C 1 C 2 C 3<br />
AC-2 AC-3 AC-4<br />
0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 2,000 10,000<br />
1,000 5,000<br />
AC-1 zone: Imperceptible<br />
AC-2 zone: Perceptible<br />
AC-3 zone : Reversible effects: muscular contraction<br />
AC-4 zone: Possibility <strong>of</strong> irreversible effects<br />
AC-4-1 zone: Up to 5%probability <strong>of</strong> heart fibrillation<br />
AC-4-2 zone: Up to 50% probability <strong>of</strong> heart fibrillation<br />
AC-4-3 zone: More than 50% probability <strong>of</strong> heart fibrillation<br />
AC-4.1 AC-4.2<br />
AC-4.3<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
A curve: Threshold <strong>of</strong> perception <strong>of</strong> current<br />
B curve: Threshold <strong>of</strong> muscular reactions<br />
Body current<br />
I s (mA)<br />
C 1 curve: Threshold <strong>of</strong> 0% probability <strong>of</strong> ventricular<br />
fibrillation<br />
C 2 curve: Threshold <strong>of</strong> 5% probability <strong>of</strong> ventricular<br />
fibrillation<br />
C 3 curve: Threshold <strong>of</strong> 50% probability <strong>of</strong> ventricular<br />
fibrillation
F - Protection against electric shock<br />
Two measures <strong>of</strong> protection against direct<br />
contact hazards are <strong>of</strong>ten required, since, in<br />
practice, the first measure may not be infallible<br />
Standards and regulations distinguish two kinds<br />
<strong>of</strong> dangerous contact,<br />
b Direct contact<br />
b Indirect contact<br />
and corresponding protective measures<br />
Is<br />
Is: Touch current<br />
1 2 3 N<br />
Busbars<br />
<strong>General</strong><br />
.2 Protection against electric shock<br />
The fundamental rule <strong>of</strong> protection against electric shock is provided by the<br />
document IEC 61140 which covers both <strong>electrical</strong> <strong>installation</strong>s and <strong>electrical</strong><br />
equipment.<br />
Hazardous-live-parts shall not be accessible and accessible conductive parts shall<br />
not be hazardous.<br />
This requirement needs to apply under:<br />
b Normal conditions, and<br />
b Under a single fault condition<br />
Various measures are adopted to protect against this hazard, and include:<br />
b Automatic disconnection <strong>of</strong> the power supply to the connected <strong>electrical</strong> equipment<br />
b Special arrangements such as:<br />
v The use <strong>of</strong> class II insulation materials, or an equivalent level <strong>of</strong> insulation<br />
v Non-conducting location, out <strong>of</strong> arm’s reach or interposition <strong>of</strong> barriers<br />
v Equipotential bonding<br />
v Electrical separation by means <strong>of</strong> isolating transformers<br />
.3 Direct and indirect contact<br />
Direct contact<br />
A direct contact refers to a person coming into contact with a conductor which is live<br />
in normal circumstances (see Fig. F2).<br />
IEC 61140 standard has renamed “protection against direct contact” with the term<br />
“basic protection”. The former name is at least kept for information.<br />
Indirect contact<br />
Fig. F2 : Direct contact Fig F3 : Indirect contact<br />
An indirect contact refers to a person coming into contact with an exposedconductive-part<br />
which is not normally alive, but has become alive accidentally (due<br />
to insulation failure or some other cause).<br />
The fault current raise the exposed-conductive-part to a voltage liable to be<br />
hazardous which could be at the origin <strong>of</strong> a touch current through a person coming<br />
into contact with this exposed-conductive-part (see Fig. F3).<br />
IEC 61140 standard has renamed “protection against indirect contact” with the term<br />
“fault protection”. The former name is at least kept for information.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Is<br />
1 2 3 PE<br />
Id<br />
Insulation<br />
failure<br />
Id: Insulation fault current<br />
F3<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
F<br />
F - Protection against electric shock<br />
IEC and national standards frequently<br />
distinguish two protection:<br />
b Complete (insulation, enclosures)<br />
b Partial or particular<br />
Fig. F5 : Example <strong>of</strong> isolation by envelope<br />
2 Protection against direct contact<br />
Two complementary measures are commonly used as protection against the<br />
dangers <strong>of</strong> direct contact:<br />
b The physical prevention <strong>of</strong> contact with live parts by barriers, insulation,<br />
inaccessibility, etc.<br />
b Additional protection in the event that a direct contact occurs, despite or due to<br />
failure <strong>of</strong> the above measures. This protection is based on residual-current operating<br />
device with a high sensitivity (IΔn y 30 mA) and a low operating time. These devices<br />
are highly effective in the majority <strong>of</strong> case <strong>of</strong> direct contact.<br />
2.1 Measures <strong>of</strong> protection against direct contact<br />
Protection by the insulation <strong>of</strong> live parts<br />
This protection consists <strong>of</strong> an insulation which complies with the relevant standards<br />
(see Fig. F ). Paints, lacquers and varnishes do not provide an adequate protection.<br />
Fig. F4 : Inherent protection against direct contact by insulation <strong>of</strong> a 3-phase cable with outer<br />
sheath<br />
Protection by means <strong>of</strong> barriers or enclosures<br />
This measure is in widespread use, since many components and materials are<br />
installed in cabinets, assemblies, control panels and distribution boards (see Fig. F5).<br />
To be considered as providing effective protection against direct contact hazards,<br />
these equipment must possess a degree <strong>of</strong> protection equal to at least IP 2X or<br />
IP XXB (see chapter E sub-clause 3.4).<br />
Moreover, an opening in an enclosure (door, front panel, drawer, etc.) must only be<br />
removable, open or withdrawn:<br />
b By means <strong>of</strong> a key or tool provided for this purpose, or<br />
b After complete isolation <strong>of</strong> the live parts in the enclosure, or<br />
b With the automatic interposition <strong>of</strong> another screen removable only with a key or<br />
a tool. The metal enclosure and all metal removable screen must be bonded to the<br />
protective earthing conductor <strong>of</strong> the <strong>installation</strong>.<br />
Partial measures <strong>of</strong> protection<br />
b Protection by means <strong>of</strong> obstacles, or by placing out <strong>of</strong> arm’s reach<br />
This protection is reserved only to locations to which skilled or instructed<br />
persons only have access. The erection <strong>of</strong> this protective measure is detailed in<br />
IEC 60364-4-41.<br />
Particular measures <strong>of</strong> protection<br />
b Protection by use <strong>of</strong> extra-low voltage SELV (Safety Extra-Low Voltage) or by<br />
limitation <strong>of</strong> the energy <strong>of</strong> discharge.<br />
These measures are used only in low-power circuits, and in particular circumstances,<br />
as described in section 3.5.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
An additional measure <strong>of</strong> protection against<br />
the hazards <strong>of</strong> direct contact is provided by the<br />
use <strong>of</strong> residual current operating device, which<br />
operate at 30 mA or less, and are referred to as<br />
RCDs <strong>of</strong> high sensitivity<br />
Fig. F6 : High sensitivity RCD<br />
2 Protection against direct contact<br />
2.2 Additional measure <strong>of</strong> protection against direct<br />
contact<br />
All the preceding protective measures are preventive, but experience has shown<br />
that for various reasons they cannot be regarded as being infallible. Among these<br />
reasons may be cited:<br />
b Lack <strong>of</strong> proper maintenance<br />
b Imprudence, carelessness<br />
b Normal (or abnormal) wear and tear <strong>of</strong> insulation; for instance flexure and abrasion<br />
<strong>of</strong> connecting leads<br />
b Accidental contact<br />
b Immersion in water, etc. A situation in which insulation is no longer effective<br />
In order to protect users in such circumstances, highly sensitive fast tripping<br />
devices, based on the detection <strong>of</strong> residual currents to earth (which may or may<br />
not be through a human being or animal) are used to disconnect the power<br />
supply automatically, and with sufficient rapidity to prevent injury to, or death by<br />
electrocution, <strong>of</strong> a normally healthy human being (see Fig. F6).<br />
These devices operate on the principle <strong>of</strong> differential current measurement, in which<br />
any difference between the current entering a circuit and that leaving it (on a system<br />
supplied from an earthed source) be flowing to earth, either through faulty insulation<br />
or through contact <strong>of</strong> an earthed part, such as a person, with a live conductor.<br />
Standardised residual-current devices, referred to as RCDs, sufficiently sensitive for<br />
protection against direct contact are rated at 30 mA <strong>of</strong> differential current.<br />
According to IEC 60364-4-41, additional protection by means <strong>of</strong> high sensitivity<br />
RCDs (I∆n y 30 mA) must be provided for circuits supplying socket-outlets with a<br />
rated current y 20 A in all locations, and for circuits supplying mobile equipment with<br />
a rated current y 32 A for use outdoors.<br />
This additional protection is required in certain countries for circuits supplying socketoutlets<br />
rated up to 32 A, and even higher if the location is wet and/or temporary<br />
(such as work sites for instance).<br />
It is also recommended to limit the number <strong>of</strong> socket-outlets protected by a RCD<br />
(e.g. 10 socket-outlets for one RCD).<br />
<strong>Chapter</strong> P section 3 itemises various common locations in which RCDs <strong>of</strong><br />
high sensitivity are obligatory (in some countries), but in any case, are highly<br />
recommended as an effective protection against both direct and indirect contact<br />
hazards.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
F5<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
F<br />
F - Protection against electric shock<br />
Protection against indirect contact hazards<br />
can be achieved by automatic disconnection <strong>of</strong><br />
the supply if the exposed-conductive-parts <strong>of</strong><br />
equipment are properly earthed<br />
(1) Touch voltage Uc is the voltage existing (as the result <strong>of</strong><br />
insulation failure) between an exposed-conductive-part and<br />
any conductive element within reach which is at a different<br />
(generally earth) potential.<br />
3 Protection against indirect<br />
contact<br />
Exposed-conductive-parts used in the manufacturing process <strong>of</strong> an <strong>electrical</strong><br />
equipment is separated from the live parts <strong>of</strong> the equipment by the “basic insulation”.<br />
Failure <strong>of</strong> the basic insulation will result in the exposed-conductive-parts being alive.<br />
Touching a normally dead part <strong>of</strong> an <strong>electrical</strong> equipment which has become live due<br />
to the failure <strong>of</strong> its insulation, is referred to as an indirect contact.<br />
3.1 Measures <strong>of</strong> protection: two levels<br />
Two levels <strong>of</strong> protective measures exist:<br />
b 1 st level: The earthing <strong>of</strong> all exposed-conductive-parts <strong>of</strong> <strong>electrical</strong> equipment in the<br />
<strong>installation</strong> and the constitution <strong>of</strong> an equipotential bonding network (see chapter G<br />
section 6).<br />
b 2 sd level: Automatic disconnection <strong>of</strong> the supply <strong>of</strong> the section <strong>of</strong> the <strong>installation</strong><br />
concerned, in such a way that the touch-voltage/time safety requirements are<br />
respected for any level <strong>of</strong> touch voltage Uc (1) (see Fig. F7).<br />
Earth<br />
connection<br />
Fig. F7 : Illustration <strong>of</strong> the dangerous touch voltage Uc<br />
The greater the value <strong>of</strong> Uc, the greater the rapidity <strong>of</strong> supply disconnection required<br />
to provide protection (see Fig. F8). The highest value <strong>of</strong> Uc that can be tolerated<br />
indefinitely without danger to human beings is 50 V a.c.<br />
Reminder <strong>of</strong> the theoretical disconnecting-time limits<br />
Uo (V) 50 < Uo y 120 120 < Uo y 230 230 < Uo y 400 Uo > 400<br />
System TN or IT 0.8 0.4 0.2 0.1<br />
TT 0.3 0.2 0.07 0.04<br />
Fig. F8 : Maximum safe duration <strong>of</strong> the assumed values <strong>of</strong> AC touch voltage (in seconds)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Uc
F - Protection against electric shock<br />
Automatic disconnection for TT system is<br />
achieved by RCD having a sensitivity <strong>of</strong><br />
I�n i<br />
R<br />
50<br />
where R A is the resistance <strong>of</strong> the<br />
A<br />
<strong>installation</strong> earth electrode<br />
R n = 10 Ω<br />
Substation<br />
earth<br />
electrode<br />
R A = 20 Ω<br />
Installation<br />
earth<br />
electrode<br />
U f<br />
1<br />
2<br />
3<br />
N<br />
PE<br />
Fig. F9 : Automatic disconnection <strong>of</strong> supply for TT system<br />
3 Protection against indirect<br />
contact<br />
3.2 Automatic disconnection for TT system<br />
Principle<br />
In this system all exposed-conductive-parts and extraneous-conductive-parts <strong>of</strong><br />
the <strong>installation</strong> must be connected to a common earth electrode. The neutral point<br />
<strong>of</strong> the supply system is normally earthed at a pint outside the influence area <strong>of</strong> the<br />
<strong>installation</strong> earth electrode, but need not be so. The impedance <strong>of</strong> the earth-fault loop<br />
therefore consists mainly in the two earth electrodes (i.e. the source and <strong>installation</strong><br />
electrodes) in series, so that the magnitude <strong>of</strong> the earth fault current is generally<br />
too small to operate overcurrent relay or fuses, and the use <strong>of</strong> a residual current<br />
operated device is essential.<br />
This principle <strong>of</strong> protection is also valid if one common earth electrode only is used,<br />
notably in the case <strong>of</strong> a consumer-type substation within the <strong>installation</strong> area, where<br />
space limitation may impose the adoption <strong>of</strong> a TN system earthing, but where all<br />
other conditions required by the TN system cannot be fulfilled.<br />
Protection by automatic disconnection <strong>of</strong> the supply used in TT system is by RCD <strong>of</strong><br />
sensitivity: I�n i<br />
R<br />
50<br />
where R<br />
A<br />
where<br />
<strong>installation</strong> earth electrode<br />
R A is the resistance <strong>of</strong> the earth electrode for the <strong>installation</strong><br />
I Δn is the rated residual operating current <strong>of</strong> the RCD<br />
For temporary supplies (to work sites, …) and agricultural and horticultural premises,<br />
the value <strong>of</strong> 50 V is replaced by 25 V.<br />
Example (see Fig. F9)<br />
b The resistance <strong>of</strong> the earth electrode <strong>of</strong> substation neutral Rn is 10 Ω.<br />
b The resistance <strong>of</strong> the earth electrode <strong>of</strong> the <strong>installation</strong> RA is 20 Ω.<br />
b The earth-fault loop current Id = 7.7 A.<br />
b The fault voltage Uf = Id x RA = 154 V and therefore dangerous, but<br />
IΔn = 50/20 = 2.5 A so that a standard 300 mA RCD will operate in about 30 ms<br />
without intentional time delay and will clear the fault where a fault voltage exceeding<br />
appears on an exposed-conductive-part.<br />
Fig. F10 : Maximum disconnecting time for AC final circuits not exceeding 32 A<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Uo<br />
(1) Uo is the nominal phase to earth voltage<br />
(1) (V) T (s)<br />
50 < Uo y 120 0.3<br />
120 < Uo y 230 0.2<br />
230 < Uo y 400 0.07<br />
Uo > 400 0.04<br />
Specified maximum disconnection time<br />
The tripping times <strong>of</strong> RCDs are generally lower than those required in the majority<br />
<strong>of</strong> national standards; this feature facilitates their use and allows the adoption <strong>of</strong> an<br />
effective discriminative protection.<br />
The IEC 60364-4-41 specifies the maximum operating time <strong>of</strong> protective devices<br />
used in TT system for the protection against indirect contact:<br />
b For all final circuits with a rated current not exceeding 32 A, the maximum<br />
disconnecting time will not exceed the values indicated in Figure F10<br />
b For all other circuits, the maximum disconnecting time is fixed to 1s. This limit<br />
enables discrimination between RCDs when installed on distribution circuits.<br />
RCD is a general term for all devices operating on the residual-current principle.<br />
RCCB (Residual Current Circuit-Breaker) as defined in IEC 61008 series is a specific<br />
class <strong>of</strong> RCD.<br />
Type G (general) and type S (Selective) <strong>of</strong> IEC 61008 have a tripping time/current<br />
characteristics as shown in Figure F11 next page. These characteristics allow a certain<br />
degree <strong>of</strong> selective tripping between the several combination <strong>of</strong> ratings and types, as<br />
shown later in sub-clause 4.3. Industrial type RCD according to IEC 60947-2 provide<br />
more possibilities <strong>of</strong> discrimination due to their flexibility <strong>of</strong> time-delaying.<br />
F7<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
F8<br />
F - Protection against electric shock<br />
The automatic disconnection for TN system is<br />
achieved by overcurrent protective devices or<br />
RCD’s<br />
N<br />
A<br />
F<br />
35 mm 2<br />
Fig. F12 : Automatic disconnection in TN system<br />
E<br />
B<br />
D C<br />
1<br />
2<br />
3<br />
PEN<br />
NS160<br />
50 m<br />
35 mm 2<br />
U f<br />
3 Protection against indirect<br />
contact<br />
3.3 Automatic disconnection for TN systems<br />
Principle<br />
x I Δn 1 2 5 > 5<br />
Domestic Instantaneous 0.3 0.15 0.04 0.04<br />
Fig. F11 : Maximum operating time <strong>of</strong> RCD’s (in seconds)<br />
In this system all exposed and extraneous-conductive-parts <strong>of</strong> the <strong>installation</strong> are<br />
connected directly to the earthed point <strong>of</strong> the power supply by protective conductors.<br />
As noted in <strong>Chapter</strong> E Sub-clause 1.2, the way in which this direct connection is<br />
carried out depends on whether the TN-C, TN-S, or TN-C-S method <strong>of</strong> implementing<br />
the TN principle is used. In figure F12 the method TN-C is shown, in which the<br />
neutral conductor acts as both the Protective-Earth and Neutral (PEN) conductor. In<br />
all TN systems, any insulation fault to earth results in a phase to neutral short-circuit.<br />
High fault current levels allow to use overcurrent protection but can give rise to touch<br />
voltages exceeding 50% <strong>of</strong> the phase to neutral voltage at the fault position during<br />
the short disconnection time.<br />
In practice for utility distribution network, earth electrodes are normally installed at<br />
regular intervals along the protective conductor (PE or PEN) <strong>of</strong> the network, while<br />
the consumer is <strong>of</strong>ten required to install an earth electrode at the service entrance.<br />
On large <strong>installation</strong>s additional earth electrodes dispersed around the premises are<br />
<strong>of</strong>ten provided, in order to reduce the touch voltage as much as possible. In high-rise<br />
apartment blocks, all extraneous conductive parts are connected to the protective<br />
conductor at each level. In order to ensure adequate protection, the earth-fault<br />
current<br />
Id or 0.8 I<br />
Uo<br />
=<br />
Zc<br />
Uo<br />
u must be higher or equal to Ia, where:<br />
Zs<br />
b Uo = nominal phase to neutral voltage<br />
b Id = the fault current<br />
b Ia = current equal to the value required to operate the protective device in the time<br />
specified<br />
b Zs = earth-fault current loop impedance, equal to the sum <strong>of</strong> the impedances <strong>of</strong> the<br />
source, the live phase conductors to the fault position, the protective conductors from<br />
the fault position back to the source<br />
b Zc = the faulty-circuit loop impedance (see “conventional method” Sub-clause 6.2)<br />
Note: The path through earth electrodes back to the source will have (generally)<br />
much higher impedance values than those listed above, and need not be considered.<br />
Example (see Fig. F12)<br />
230<br />
The fault voltage Uf = = 115 V and and is is hazardous;<br />
2<br />
The fault loop impedance Zs=Zab + Zbc + Zde + Zen + Zna.<br />
If Zbc and Zde are predominant, then:<br />
L<br />
“conventional Zs = 2� = 64method” . 3 m� , , so and so that in this example will give an estimated fault current <strong>of</strong><br />
S<br />
230<br />
Id=<br />
= (<br />
64.3 x10-3 3,576 A (≈ 22 In based on a NS 160 circuit-breaker).<br />
The “instantaneous” magnetic trip unit adjustment <strong>of</strong> the circuit-breaker is many time<br />
less than this short-circuit value, so that positive operation in the shortest possible<br />
time is assured.<br />
Note: Some authorities base such calculations on the assumption that a voltage<br />
drop <strong>of</strong> 20% occurs in the part <strong>of</strong> the impedance loop BANE.<br />
This method, which is recommended, is explained in chapter F sub-clause 6.2<br />
“conventional<br />
“conventional<br />
method”<br />
method”<br />
and<br />
and<br />
in<br />
in<br />
this<br />
this<br />
example<br />
example<br />
will<br />
will<br />
give<br />
give<br />
an<br />
an<br />
estimated<br />
estimated<br />
fault<br />
fault<br />
current<br />
current<br />
<strong>of</strong><br />
<strong>of</strong><br />
3<br />
230 x 0.8 x 10<br />
= 2,816 A ( (≈ 18 In).<br />
64.3<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Type S 0.5 0.2 0.15 0.15<br />
Industrial Instantaneous 0.3 0.15 0.04 0.04<br />
Time-delay (0.06) 0.5 0.2 0.15 0.15<br />
Time-delay (other) According to manufacturer
F - Protection against electric shock<br />
If the protection is to be provided by a circuitbreaker,<br />
it is sufficient to verify that the fault<br />
current will always exceed the current-setting<br />
level <strong>of</strong> the instantaneous or short-time delay<br />
tripping unit (Im)<br />
t<br />
1: Short-time delayed trip<br />
2: Instantaneous trip<br />
Im Uo/Zs<br />
1<br />
2<br />
I<br />
3 Protection against indirect<br />
contact<br />
Specified maximum disconnection time<br />
The IEC 60364-4-41 specifies the maximum operating time <strong>of</strong> protective devices<br />
used in TN system for the protection against indirect contact:<br />
b For all final circuits with a rated current not exceeding 32 A, the maximum<br />
disconnecting time will not exceed the values indicated in Figure F13<br />
b For all other circuits, the maximum disconnecting time is fixed to 5s. This limit<br />
enables discrimination between protective devices installed on distribution circuits<br />
Note: The use <strong>of</strong> RCDs may be necessary on TN-earthed systems. Use <strong>of</strong> RCDs on<br />
TN-C-S systems means that the protective conductor and the neutral conductor must<br />
(evidently) be separated upstream <strong>of</strong> the RCD. This separation is commonly made at<br />
the service entrance.<br />
Fig. F13 : Maximum disconnecting time for AC final circuits not exceeding 32 A<br />
tc = 0.4 s<br />
Fig. F14 : Disconnection by circuit-breaker for a TN system Fig. F15 : Disconnection by fuses for a TN system<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Uo<br />
(1) Uo is the nominal phase to earth voltage<br />
(1) (V) T (s)<br />
50 < Uo y 120 0.8<br />
120 < Uo y 230 0.4<br />
230 < Uo y 400 0.2<br />
Uo > 400 0.1<br />
Protection by means <strong>of</strong> circuit-breaker (see Fig. F14)<br />
The instantaneous trip unit <strong>of</strong> a circuit-breaker will eliminate a short-circuit to earth in<br />
less than 0.1 second.<br />
In consequence, automatic disconnection within the maximum allowable time will<br />
always be assured, since all types <strong>of</strong> trip unit, magnetic or electronic, instantaneous<br />
or slightly retarded, are suitable: Ia = Im. The maximum tolerance authorised<br />
by the relevant standard, however, must always be taken into consideration. It is<br />
sufficient therefore that the fault current Uo Uo<br />
or 0.8 determined by by calculation (or estimated<br />
Zs Zc<br />
(or estimated on site) on site) be greater be greater than than the instantaneous the instantaneous trip-setting trip-setting current, current, or than or than the very short-<br />
the very short-time tripping threshold level, to be sure <strong>of</strong> tripping within the permitted<br />
time limit.<br />
Ia can be determined from the fuse<br />
Protection by means <strong>of</strong> fuses (see Fig. F15)<br />
performance curve. In any case, protection The value <strong>of</strong> current which assures the correct operation <strong>of</strong> a fuse can be<br />
cannot be achieved if the loop impedance Zs ascertained from a current/time performance graph for the fuse concerned.<br />
or Zc exceeds a certain value<br />
therefore that The the fault current Uo Uo<br />
or 0.8 determined as determined by calculation above, must (or largely estimated exceed that<br />
Zs Zc<br />
on site) be greater necessary than the to ensure instantaneous positive operation trip-setting operation <strong>of</strong> current, the fuse. or The than condition the very short- to observe<br />
therefore is that Ia < Uo Uo<br />
or 0.8 as as indicated in Figure in Figure F15. F15.<br />
Zs Zc<br />
t<br />
Ia Uo/Zs<br />
I<br />
F9<br />
© Schneider Electric - all rights reserved
F10<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
In IT system the first fault to earth should not<br />
cause any disconnection<br />
Fig. F16 : Phases to earth insulation monitoring device<br />
obligatory in IT system<br />
(1) Resistive leakage current to earth through the insulation is<br />
assumed to be negligibly small in the example.<br />
3 Protection against indirect<br />
contact<br />
Example: The nominal phase to neutral voltage <strong>of</strong> the network is 230 V and<br />
the maximum disconnection time given by the graph in Figure F15 is 0.4 s.<br />
The corresponding value <strong>of</strong> Ia can be read from the graph. Using the voltage (230 V)<br />
and the current Ia,<br />
Ia,<br />
the<br />
the<br />
complete<br />
complete<br />
loop<br />
loop<br />
impedance<br />
impedance<br />
or<br />
or<br />
the<br />
the<br />
circuit<br />
circuit<br />
loop<br />
loop<br />
impedance<br />
impedance<br />
can<br />
can<br />
230<br />
be calculated from Zs = or Zc = 0.8<br />
Ia Ia<br />
230 . This impedance value must never be<br />
exceeded and should preferably be substantially less to ensure satisfactory fuse<br />
operation.<br />
Protection by means <strong>of</strong> Residual Current Devices for<br />
TN-S circuits<br />
Residual Current Devices must be used where:<br />
b The loop impedance cannot be determined precisely (lengths difficult to estimate,<br />
presence <strong>of</strong> metallic material close to the wiring)<br />
b The fault current is so low that the disconnecting time cannot be met by using<br />
overcurrent protective devices<br />
The rated tripping current <strong>of</strong> RCDs being in the order <strong>of</strong> a few amps, it is well below<br />
the fault current level. RCDs are consequently well adapted to this situation.<br />
In practice, they are <strong>of</strong>ten installed in the LV sub distribution and in many countries,<br />
the automatic disconnection <strong>of</strong> final circuits shall be achieved by Residual Current<br />
Devices.<br />
3.4 Automatic disconnection on a second fault in an<br />
IT system<br />
In this type <strong>of</strong> system:<br />
b The <strong>installation</strong> is isolated from earth, or the neutral point <strong>of</strong> its power-supply<br />
source is connected to earth through a high impedance<br />
b All exposed and extraneous-conductive-parts are earthed via an <strong>installation</strong> earth<br />
electrode.<br />
First fault situation<br />
On the occurrence <strong>of</strong> a true fault to earth, referred to as a “first fault”, the fault current<br />
is very low, such that the rule Id x RA y 50 V (see F3.2) is fulfilled and no dangerous<br />
fault voltages can occur.<br />
In practice the current Id is low, a condition that is neither dangerous to personnel,<br />
nor harmful to the <strong>installation</strong>.<br />
However, in this system:<br />
b A permanent monitoring <strong>of</strong> the insulation to earth must be provided, coupled with<br />
an alarm signal (audio and/or flashing lights, etc.) operating in the event <strong>of</strong> a first<br />
earth fault (see Fig. F1 )<br />
b The rapid location and repair <strong>of</strong> a first fault is imperative if the full benefits <strong>of</strong> the<br />
IT system are to be realised. Continuity <strong>of</strong> service is the great advantage afforded by<br />
the system.<br />
For a network formed from 1 km <strong>of</strong> new conductors, the leakage (capacitive)<br />
impedance to earth Zf is <strong>of</strong> the order <strong>of</strong> 3,500 Ω per phase. In normal operation, the<br />
capacitive current (1) to earth is therefore:<br />
Uo 230<br />
= = 66 mA per phase.<br />
Zf 3,500<br />
During a phase to earth fault, as indicated in Figure F17 opposite page, the current<br />
passing through the electrode resistance RnA is the vector sum <strong>of</strong> the capacitive<br />
currents in the two healthy phases. The voltages <strong>of</strong> the healthy phases have<br />
(because <strong>of</strong> the fault) increased to 3 the normal phase voltage, so that the capacitive<br />
currents increase by the same amount. These currents are displaced, one from the<br />
other by 60°, so that when added vectorially, this amounts to 3 x 66 mA = 198 mA, in<br />
the present example.<br />
The fault voltage Uf is therefore equal to 198 x 5 x 10-3 = 0.99 V, which is obviously<br />
harmless.<br />
The current through the short-circuit to earth is given by the vector sum <strong>of</strong> the<br />
neutral-resistor current Id1 (=153 mA) and the capacitive current Id2 (198 mA).<br />
Since the exposed-conductive-parts <strong>of</strong> the <strong>installation</strong> are connected directly to<br />
earth, the neutral impedance Zct plays practically no part in the production <strong>of</strong> touch<br />
voltages to earth.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
The simultaneous existence <strong>of</strong> two earth faults<br />
(if not both on the same phase) is dangerous,<br />
and rapid clearance by fuses or automatic<br />
circuit-breaker tripping depends on the type <strong>of</strong><br />
earth-bonding scheme, and whether separate<br />
earthing electrodes are used or not, in the<br />
<strong>installation</strong> concerned<br />
(1) Based on the “conventional method” noted in the first<br />
example <strong>of</strong> Sub-clause 3.3.<br />
3 Protection against indirect<br />
contact<br />
Z ct = 1,500 Ω<br />
R nA = 5 Ω<br />
Id1<br />
Fig. F17 : Fault current path for a first fault in IT system<br />
Second fault situation<br />
On the appearance <strong>of</strong> a second fault, on a different phase, or on a neutral conductor,<br />
a rapid disconnection becomes imperative. Fault clearance is carried out differently in<br />
each <strong>of</strong> the following cases:<br />
1 st case<br />
It concerns an <strong>installation</strong> in which all exposed conductive parts are bonded to a<br />
common PE conductor, as shown in Figure F18.<br />
In this case no earth electrodes are included in the fault current path, so that a high<br />
level <strong>of</strong> fault current is assured, and conventional overcurrent protective devices are<br />
used, i.e. circuit-breakers and fuses.<br />
The first fault could occur at the end <strong>of</strong> a circuit in a remote part <strong>of</strong> the <strong>installation</strong>,<br />
while the second fault could feasibly be located at the opposite end <strong>of</strong> the <strong>installation</strong>.<br />
For this reason, it is conventional to double the loop impedance <strong>of</strong> a circuit, when<br />
calculating the anticipated fault setting level for its overcurrent protective device(s).<br />
Where the system includes a neutral conductor in addition to the 3 phase<br />
conductors, the lowest short-circuit fault currents will occur if one <strong>of</strong> the (two) faults is<br />
from the neutral conductor to earth (all four conductors are insulated from earth in an<br />
IT scheme). In four-wire IT <strong>installation</strong>s, therefore, the phase-to-neutral voltage must<br />
be used to calculate short-circuit protective levels i.e. 0.8 Uo<br />
u I a (1) where<br />
2 Zc<br />
Uo = phase to neutral voltage<br />
Zc = impedance <strong>of</strong> the circuit fault-current loop (see F3.3)<br />
Ia = current level for trip setting<br />
If no neutral conductor is distributed, then the voltage to use for the fault-current<br />
calculation is the phase-to-phase value, i.e.<br />
calculation is the phase-to-phase value, i.e. 0.8<br />
b Maximum tripping times<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Ω<br />
Id2<br />
3 Uo<br />
2 Zc<br />
(1)<br />
(1) u I a<br />
Id1 + Id2<br />
Disconnecting times for IT system depends on how the different <strong>installation</strong> and<br />
substation earth electrodes are interconnected.<br />
1<br />
2<br />
3<br />
N<br />
PE<br />
For final circuits supplying <strong>electrical</strong> equipment with a rated current not exceeding<br />
32 A and having their exposed-conductive-parts bonded with the substation earth<br />
electrode, the maximum tripping time is given in table F8. For the other circuits<br />
within the same group <strong>of</strong> interconnected exposed-conductive-parts, the maximum<br />
disconnecting time is 5 s. This is due to the fact that any double fault situation within<br />
this group will result in a short-circuit current as in TN system.<br />
For final circuits supplying <strong>electrical</strong> equipment with a rated current not exceeding<br />
32 A and having their exposed-conductive-parts connected to an independent earth<br />
electrode <strong>electrical</strong>ly separated from the substation earth electrode, the maximum<br />
tripping time is given in Figure F13. For the other circuits within the same group <strong>of</strong><br />
non interconnected exposed-conductive-parts, the maximum disconnecting time is<br />
1s. This is due to the fact that any double fault situation resulting from one insulation<br />
fault within this group and another insulation fault from another group will generate a<br />
fault current limited by the different earth electrode resistances as in TT system.<br />
Zf<br />
B<br />
U f<br />
F11<br />
© Schneider Electric - all rights reserved
F12<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
3 Protection against indirect<br />
contact<br />
Fig. F18 : Circuit-breaker tripping on double fault situation when exposed-conductive-parts are<br />
connected to a common protective conductor<br />
b Protection by circuit-breaker<br />
In the case shown in Figure F18, the adjustments <strong>of</strong> instantaneous and short-time<br />
delay overcurrent trip unit must be decided. The times recommended here above can<br />
be readily complied with. The short-circuit protection provided by the NS 160 circuitbreaker<br />
is suitable to clear a phase to phase short-circuit occurring at the load ends<br />
<strong>of</strong> the circuits concerned.<br />
Reminder: In an IT system, the two circuits involved in a phase to phase short-circuit<br />
are assumed to be <strong>of</strong> equal length, with the same cross sectional area conductors,<br />
the PE conductors being the same cross sectional area as the phase conductors. In<br />
such a case, the impedance <strong>of</strong> the circuit loop when using the “conventional method”<br />
(sub clause 6.2) will be twice that calculated for one <strong>of</strong> the circuits in the TN case,<br />
shown in <strong>Chapter</strong> F sub clause 3.3.<br />
L<br />
So that The the resistance resistance <strong>of</strong> <strong>of</strong> circuit 1 loop FGHJ = 22R RJH JH = 2�<br />
in m�<br />
where: where:<br />
a<br />
ρ = resistance <strong>of</strong> copper rod 1 meter long <strong>of</strong> cross sectional area 1 mm2 , in mΩ<br />
L = length <strong>of</strong> the circuit in meters<br />
a = cross sectional area <strong>of</strong> the conductor in mm2 FGHJ = 2 x 22.5 x 50/35 = 64.3 mΩ<br />
and the loop resistance B, C, D, E, F, G, H, J will be 2 x 64.3 = 129 mΩ.<br />
The fault current will therefore be 0.8 x 3 x 230 x 10 3 /129 = 2,470 A.<br />
b Protection by fuses<br />
The current I a for which fuse operation must be assured in a time specified according<br />
to here above can be found from fuse operating curves, as described in figure F15.<br />
The current indicated should be significantly lower than the fault currents calculated<br />
for the circuit concerned.<br />
b Protection by Residual current circuit-breakers (RCCBs)<br />
For low values <strong>of</strong> short-circuit current, RCCBs are necessary. Protection against<br />
indirect contact hazards can be achieved then by using one RCCB for each circuit.<br />
2 nd case<br />
b It concerns exposed conductive parts which are earthed either individually (each part<br />
having its own earth electrode) or in separate groups (one electrode for each group).<br />
If all exposed conductive parts are not bonded to a common electrode system, then<br />
it is possible for the second earth fault to occur in a different group or in a separately<br />
earthed individual apparatus. Additional protection to that described above for<br />
case 1, is required, and consists <strong>of</strong> a RCD placed at the circuit-breaker controlling<br />
each group and each individually-earthed apparatus.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
K<br />
A<br />
Id<br />
R A<br />
NS160<br />
160 A<br />
50 m<br />
35 mm2 F<br />
G<br />
J<br />
H<br />
E<br />
D<br />
B<br />
C<br />
1<br />
2<br />
3<br />
N<br />
PE<br />
50 m<br />
35 mm 2
F - Protection against electric shock<br />
Case 1<br />
Rn<br />
RCD<br />
Ω<br />
N<br />
PIM<br />
RA<br />
Fig. F19 : Correspondence between the earth leakage capacitance and the first fault current<br />
Group<br />
earth<br />
Case 2<br />
Fig. F20 : Application <strong>of</strong> RCDs when exposed-conductive-parts are earthed individually or by group on IT system<br />
Extra-low voltage is used where the risks<br />
are great: swimming pools, wandering-lead<br />
hand lamps, and other portable appliances for<br />
outdoor use, etc.<br />
3 Protection against indirect<br />
contact<br />
The reason for this requirement is that the separate-group electrodes are “bonded”<br />
through the earth so that the phase to phase short-circuit current will generally be<br />
limited when passing through the earth bond by the electrode contact resistances<br />
with the earth, thereby making protection by overcurrent devices unreliable. The<br />
more sensitive RCDs are therefore necessary, but the operating current <strong>of</strong> the RCDs<br />
must evidently exceed that which occurs for a first fault (see Fig. F19).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Leakage capacitance First fault current<br />
(µF) (A)<br />
1 0.07<br />
5 0.36<br />
30 2.17<br />
Note: 1 µF is the 1 km typical leakage capacitance for<br />
4-conductor cable.<br />
For a second fault occurring within a group having a common earth-electrode<br />
system, the overcurrent protection operates, as described above for case 1.<br />
Note 1: See also <strong>Chapter</strong> G Sub-clause 7.2, protection <strong>of</strong> the neutral conductor.<br />
Note 2: In 3-phase 4-wire <strong>installation</strong>s, protection against overcurrent in the neutral<br />
conductor is sometimes more conveniently achieved by using a ring-type current<br />
transformer over the single-core neutral conductor (see Fig. F20).<br />
RCD<br />
Ω<br />
N<br />
PIM<br />
Group<br />
earth 1<br />
RCD RCD<br />
Rn RA1 RA2<br />
Group<br />
earth 2<br />
3.5 Measures <strong>of</strong> protection against direct or indirect<br />
contact without automatic disconnection <strong>of</strong> supply<br />
The use <strong>of</strong> SELV (Safety Extra-Low Voltage)<br />
Safety by extra low voltage SELV is used in situations where the operation <strong>of</strong> <strong>electrical</strong><br />
equipment presents a serious hazard (swimming pools, amusement parks, etc.).<br />
This measure depends on supplying power at extra-low voltage from the secondary<br />
windings <strong>of</strong> isolating transformers especially <strong>design</strong>ed according to national or to<br />
international (IEC 60742) standard. The impulse withstand level <strong>of</strong> insulation between<br />
the primary and secondary windings is very high, and/or an earthed metal screen<br />
is sometimes incorporated between the windings. The secondary voltage never<br />
exceeds 50 V rms.<br />
Three conditions <strong>of</strong> exploitation must be respected in order to provide satisfactory<br />
protection against indirect contact:<br />
b No live conductor at SELV must be connected to earth<br />
b Exposed-conductive-parts <strong>of</strong> SELV supplied equipment must not be connected to<br />
earth, to other exposed conductive parts, or to extraneous-conductive-parts<br />
b All live parts <strong>of</strong> SELV circuits and <strong>of</strong> other circuits <strong>of</strong> higher voltage must be<br />
separated by a distance at least equal to that between the primary and secondary<br />
windings <strong>of</strong> a safety isolating transformer.<br />
F13<br />
© Schneider Electric - all rights reserved
F14<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
The <strong>electrical</strong> separation <strong>of</strong> circuits is suitable<br />
for relatively short cable lengths and high levels<br />
<strong>of</strong> insulation resistance. It is preferably used for<br />
an individual appliance<br />
230 V/230 V<br />
Fig. F22 : Safety supply from a class II separation transformer<br />
3 Protection against indirect<br />
contact<br />
These measures require that:<br />
b SELV circuits must use conduits exclusively provided for them, unless cables which<br />
are insulated for the highest voltage <strong>of</strong> the other circuits are used for the SELV circuits<br />
b Socket outlets for the SELV system must not have an earth-pin contact. The<br />
SELV circuit plugs and sockets must be special, so that inadvertent connection to a<br />
different voltage level is not possible.<br />
Note: In normal conditions, when the SELV voltage is less than 25 V, there is no<br />
need to provide protection against direct contact hazards. Particular requirements<br />
are indicated in <strong>Chapter</strong> P, Clause 3: “special locations”.<br />
The use <strong>of</strong> PELV (Protection by Extra Low Voltage) (see Fig. F21)<br />
This system is for general use where low voltage is required, or preferred for safety<br />
reasons, other than in the high-risk locations noted above. The conception is similar<br />
to that <strong>of</strong> the SELV system, but the secondary circuit is earthed at one point.<br />
IEC 60364-4-41 defines precisely the significance <strong>of</strong> the reference PELV. Protection<br />
against direct contact hazards is generally necessary, except when the equipment<br />
is in the zone <strong>of</strong> equipotential bonding, and the nominal voltage does not exceed<br />
25 V rms, and the equipment is used in normally dry locations only, and large-area<br />
contact with the human body is not expected. In all other cases, 6 V rms is the<br />
maximum permitted voltage, where no direct contact protection is provided.<br />
230 V / 24 V<br />
Fig. F21 : Low-voltage supplies from a safety isolating transformer<br />
FELV system (Functional Extra-Low Voltage)<br />
Where, for functional reasons, a voltage <strong>of</strong> 50 V or less is used, but not all <strong>of</strong> the<br />
requirements relating to SELV or PELV are fulfilled, appropriate measures described<br />
in IEC 60364-4-41 must be taken to ensure protection against both direct and<br />
indirect contact hazards, according to the location and use <strong>of</strong> these circuits.<br />
Note: Such conditions may, for example, be encountered when the circuit contains<br />
equipment (such as transformers, relays, remote-control switches, contactors)<br />
insufficiently insulated with respect to circuits at higher voltages.<br />
The <strong>electrical</strong> separation <strong>of</strong> circuits (see Fig. F22)<br />
The principle <strong>of</strong> the <strong>electrical</strong> separation <strong>of</strong> circuits (generally single-phase circuits)<br />
for safety purposes is based on the following rationale.<br />
The two conductors from the unearthed single-phase secondary winding <strong>of</strong> a<br />
separation transformer are insulated from earth.<br />
If a direct contact is made with one conductor, a very small current only will flow into<br />
the person making contact, through the earth and back to the other conductor, via<br />
the inherent capacitance <strong>of</strong> that conductor with respect to earth. Since the conductor<br />
capacitance to earth is very small, the current is generally below the level <strong>of</strong> perception.<br />
As the length <strong>of</strong> circuit cable increases, the direct contact current will progressively<br />
increase to a point where a dangerous electric shock will be experienced.<br />
Even if a short length <strong>of</strong> cable precludes any danger from capacitive current, a low<br />
value <strong>of</strong> insulation resistance with respect to earth can result in danger, since the<br />
current path is then via the person making contact, through the earth and back to the<br />
other conductor through the low conductor-to-earth insulation resistance.<br />
For these reasons, relatively short lengths <strong>of</strong> well insulated cables are essential in<br />
separation systems.<br />
Transformers are specially <strong>design</strong>ed for this duty, with a high degree <strong>of</strong> insulation<br />
between primary and secondary windings, or with equivalent protection, such as an<br />
earthed metal screen between the windings. Construction <strong>of</strong> the transformer is to<br />
class II insulation standards.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
Symbol:<br />
In principle, safety by placing simultaneouslyaccessible<br />
conductive parts out-<strong>of</strong>-reach, or by<br />
interposing obstacles, requires also a nonconducting<br />
floor, and so is not an easily applied<br />
principle<br />
(1) It is recommended in IEC 364-4-41 that the product <strong>of</strong> the<br />
nominal voltage <strong>of</strong> the circuit in volts and length in metres <strong>of</strong><br />
the wiring system should not exceed 100,000, and that the<br />
length <strong>of</strong> the wiring system should not exceed 500 m.<br />
3 Protection against indirect<br />
contact<br />
As indicated before, successful exploitation <strong>of</strong> the principle requires that:<br />
b No conductor or exposed conductive part <strong>of</strong> the secondary circuit must be<br />
connected to earth,<br />
b The length <strong>of</strong> secondary cabling must be limited to avoid large capacitance values (1) ,<br />
b A high insulation-resistance value must be maintained for the cabling and appliances.<br />
These conditions generally limit the application <strong>of</strong> this safety measure to an<br />
individual appliance.<br />
In the case where several appliances are supplied from a separation transformer, it is<br />
necessary to observe the following requirements:<br />
b The exposed conductive parts <strong>of</strong> all appliances must be connected together by an<br />
insulated protective conductor, but not connected to earth,<br />
b The socket outlets must be provided with an earth-pin connection. The earth-pin<br />
connection is used in this case only to ensure the interconnection (bonding) <strong>of</strong> all<br />
exposed conductive parts.<br />
In the case <strong>of</strong> a second fault, overcurrent protection must provide automatic<br />
disconnection in the same conditions as those required for an IT system <strong>of</strong> power<br />
system earthing.<br />
Class II equipment<br />
These appliances are also referred to as having “double insulation” since in class<br />
II appliances a supplementary insulation is added to the basic insulation (see<br />
Fig. F23).<br />
No conductive parts <strong>of</strong> a class II appliance must be connected to a protective conductor:<br />
b Most portable or semi-fixed equipment, certain lamps, and some types <strong>of</strong><br />
transformer are <strong>design</strong>ed to have double insulation. It is important to take particular<br />
care in the exploitation <strong>of</strong> class II equipment and to verify regularly and <strong>of</strong>ten that the<br />
class II standard is maintained (no broken outer envelope, etc.). Electronic devices,<br />
radio and television sets have safety levels equivalent to class II, but are not formally<br />
class II appliances<br />
b Supplementary insulation in an <strong>electrical</strong> <strong>installation</strong>: IEC 60364-4-41(Sub-clause<br />
413-2) and some national standards such as NF C 15-100 (France) describe in<br />
more detail the necessary measures to achieve the supplementary insulation during<br />
<strong>installation</strong> work.<br />
Fig. F23 : Principle <strong>of</strong> class II insulation level<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Active part<br />
Basic insulation<br />
Supplementary insulation<br />
A simple example is that <strong>of</strong> drawing a cable into a PVC conduit. Methods are also<br />
described for distribution switchboards.<br />
b For distribution switchboards and similar equipment, IEC 60439-1 describes a set<br />
<strong>of</strong> requirements, for what is referred to as “total insulation”, equivalent to class II<br />
b Some cables are recognised as being equivalent to class II by many national standards<br />
Out-<strong>of</strong>-arm’s reach or interposition <strong>of</strong> obstacles<br />
By these means, the probability <strong>of</strong> touching a live exposed-conductive-part, while at<br />
the same time touching an extraneous-conductive-part at earth potential, is extremely<br />
low (see Fig. F24 next page). In practice, this measure can only be applied in a dry<br />
location, and is implemented according to the following conditions:<br />
b The floor and the wall <strong>of</strong> the chamber must be non-conducting, i.e. the resistance<br />
to earth at any point must be:<br />
v > 50 kΩ (<strong>installation</strong> voltage y 500 V)<br />
v > 100 kΩ (500 V < <strong>installation</strong> voltage y 1000 V)<br />
Resistance is measured by means <strong>of</strong> “MEGGER” type instruments (hand-operated<br />
generator or battery-operated electronic model) between an electrode placed on the<br />
floor or against the wall, and earth (i.e. the nearest protective earth conductor). The<br />
electrode contact area pressure must be evidently be the same for all tests.<br />
Different instruments suppliers provide electrodes specific to their own product, so<br />
that care should be taken to ensure that the electrodes used are those supplied with<br />
the instrument.<br />
F15<br />
© Schneider Electric - all rights reserved
F1<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
2.5 m<br />
Electrical<br />
apparatus<br />
Fig. F24 : Protection by out-<strong>of</strong> arm’s reach arrangements and the interposition <strong>of</strong> non-conducting obstacles<br />
Earth-free equipotential chambers are<br />
associated with particular <strong>installation</strong>s<br />
(laboratories, etc.) and give rise to a number <strong>of</strong><br />
practical <strong>installation</strong> difficulties<br />
> 2 m<br />
(1) Extraneous conductive parts entering (or leaving) the<br />
equipotential space (such as water pipes, etc.) must be<br />
encased in suitable insulating material and excluded from the<br />
equipotential network, since such parts are likely to be bonded<br />
to protective (earthed) conductors elsewhere in the <strong>installation</strong>.<br />
3 Protection against indirect<br />
contact<br />
b The placing <strong>of</strong> equipment and obstacles must be such that simultaneous contact<br />
with two exposed-conductive-parts or with an exposed conductive-part and an<br />
extraneous-conductive-part by an individual person is not possible.<br />
b No exposed protective conductor must be introduced into the chamber concerned.<br />
b Entrances to the chamber must be arranged so that persons entering are not at<br />
risk, e.g. a person standing on a conducting floor outside the chamber must not be<br />
able to reach through the doorway to touch an exposed-conductive-part, such as a<br />
lighting switch mounted in an industrial-type cast-iron conduit box, for example.<br />
Insulated floor<br />
Electrical<br />
apparatus<br />
Insulated<br />
obstacles<br />
Earth-free equipotential chambers<br />
In this scheme, all exposed-conductive-parts, including the floor (1) are bonded by<br />
suitably large conductors, such that no significant difference <strong>of</strong> potential can exist<br />
between any two points. A failure <strong>of</strong> insulation between a live conductor and the<br />
metal envelope <strong>of</strong> an appliance will result in the whole “cage” being raised to phaseto-earth<br />
voltage, but no fault current will flow. In such conditions, a person entering<br />
the chamber would be at risk (since he/she would be stepping on to a live floor).<br />
Suitable precautions must be taken to protect personnel from this danger (e.g. nonconducting<br />
floor at entrances, etc.). Special protective devices are also necessary to<br />
detect insulation failure, in the absence <strong>of</strong> significant fault current.<br />
M<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
< 2 m<br />
Insulating material<br />
Electrical<br />
apparatus<br />
Conductive<br />
floor<br />
Insulated<br />
walls<br />
Fig. F25 : Equipotential bonding <strong>of</strong> all exposed-conductive-parts simultaneously accessible
F - Protection against electric shock<br />
RCDs are very effective devices to provide<br />
protection against fire risk due to insulation<br />
fault because they can detect leakage current<br />
(ex : 300 mA) wich are too low for the other<br />
protections, but sufficient to cause a fire<br />
Beginning <strong>of</strong> fire<br />
Fig. F26 : Origin <strong>of</strong> fires in buildings<br />
Id
F18<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
4 Protection <strong>of</strong> goods<br />
in case <strong>of</strong> insulation fault<br />
Positioning GFP devices in the <strong>installation</strong><br />
Type / <strong>installation</strong> level Main-distribution Sub-distribution Comments<br />
Source Ground Return<br />
(SGR)<br />
v Used<br />
Residual Sensing (RS)<br />
(SGR)<br />
v b Often used<br />
Zero Sequence<br />
(SGR)<br />
v Possible<br />
v b Rarely used<br />
b Recommended or required<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
A<br />
RCD<br />
Fig. F29 : Distribution circuits<br />
RCD<br />
B<br />
RCD<br />
RA1 RA2<br />
Distant location<br />
5 Implementation <strong>of</strong> the TT system<br />
5.1 Protective measures<br />
Protection against indirect contact<br />
<strong>General</strong> case<br />
Protection against indirect contact is assured by RCDs, the sensitivity IΔn <strong>of</strong> which<br />
50 V<br />
complies with the condition I �n i (1) (1)<br />
RA<br />
The choice <strong>of</strong> sensitivity <strong>of</strong> the residual current device is a function <strong>of</strong> the resistance<br />
RA <strong>of</strong> the earth electrode for the <strong>installation</strong>, and is given in Figure F28.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
IΔn Maximum resistance <strong>of</strong> the earth electrode<br />
(50 V) (25 V)<br />
3 A 16 Ω 8 Ω<br />
1 A 50 Ω 25 Ω<br />
500 mA 100 Ω 50 Ω<br />
300 mA 166 Ω 83 Ω<br />
30 mA 1666 Ω 833 Ω<br />
Fig. F28 : The upper limit <strong>of</strong> resistance for an <strong>installation</strong> earthing electrode which must not be<br />
exceeded, for given sensitivity levels <strong>of</strong> RCDs at U L voltage limits <strong>of</strong> 50 V and 25 V<br />
Case <strong>of</strong> distribution circuits (see Fig. F29)<br />
IEC 60364-4-41 and a number <strong>of</strong> national standards recognize a maximum tripping<br />
time <strong>of</strong> 1 second in <strong>installation</strong> distribution circuits (as opposed to final circuits). This<br />
allows a degree <strong>of</strong> selective discrimination to be achieved:<br />
b At level A: RCD time-delayed, e.g. “S” type<br />
b At level B: RCD instantaneous<br />
Case where the exposed conductive parts <strong>of</strong> an appliance, or group <strong>of</strong><br />
appliances, are connected to a separate earth electrode (see Fig. F30)<br />
Protection against indirect contact by a RCD at the circuit-breaker level protecting<br />
each group or separately-earthed individual appliance.<br />
In each case, the sensitivity must be compatible with the resistance <strong>of</strong> the earth<br />
electrode concerned.<br />
High-sensitivity RCDs (see Fig. F31)<br />
Fig. F30 : Separate earth electrode Fig. F31 : Circuit supplying socket-outlets<br />
(1) 25 V for work-site <strong>installation</strong>s, agricultural establishments, etc.<br />
According to IEC 60364-4-41, high sensitivity RCDs (y 30 mA) must be used for<br />
protection <strong>of</strong> socket outlets with rated current y 20 A in all locations. The use <strong>of</strong> such<br />
RCDs is also recommended in the following cases:<br />
b Socket-outlet circuits in wet locations at all current ratings<br />
b Socket-outlet circuits in temporary <strong>installation</strong>s<br />
b Circuits supplying laundry rooms and swimming pools<br />
b Supply circuits to work-sites, caravans, pleasure boats, and travelling fairs<br />
See 2.2 and chapter P, section 3<br />
F19<br />
© Schneider Electric - all rights reserved
F20<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
Fig. F32 : Fire-risk location<br />
Fire-risk<br />
location<br />
5 Implementation <strong>of</strong> the TT system<br />
In high fire risk locations (see Fig. F32)<br />
RCD protection at the circuit-breaker controlling all supplies to the area at risk is<br />
necessary in some locations, and mandatory in many countries.<br />
The sensitivity <strong>of</strong> the RCD must be y 500 mA, but a 300 mA sensitivity is<br />
recommended.<br />
Protection when exposed conductive parts are not connected<br />
to earth (see Fig. F33)<br />
(In the case <strong>of</strong> an existing <strong>installation</strong> where the location is dry and provision <strong>of</strong><br />
an earthing connection is not possible, or in the event that a protective earth wire<br />
becomes broken).<br />
RCDs <strong>of</strong> high sensitivity (y 30 mA) will afford both protection against indirect-contact<br />
hazards, and the additional protection against the dangers <strong>of</strong> direct-contact.<br />
Fig. F33 : Unearthed exposed conductive parts (A)<br />
5.2 Coordination <strong>of</strong> residual current protective<br />
devices<br />
Discriminative-tripping coordination is achieved either by time-delay or by subdivision<br />
<strong>of</strong> circuits, which are then protected individually or by groups, or by a combination <strong>of</strong><br />
both methods.<br />
Such discrimination avoids the tripping <strong>of</strong> any RCD, other than that immediately<br />
upstream <strong>of</strong> a fault position:<br />
b With equipment currently available, discrimination is possible at three or four<br />
different levels <strong>of</strong> distribution :<br />
v At the main general distribution board<br />
v At local general distribution boards<br />
v At sub-distribution boards<br />
v At socket outlets for individual appliance protection<br />
b In general, at distribution boards (and sub-distribution boards, if existing) and on<br />
individual-appliance protection, devices for automatic disconnection in the event <strong>of</strong><br />
an indirect-contact hazard occurring are installed together with additional protection<br />
against direct-contact hazards.<br />
Discrimination between RCDs<br />
The general specification for achieving total discrimination between two RCDs is as<br />
follow:<br />
b The ratio between the rated residual operating currents must be > 2<br />
b Time delaying the upstream RCD<br />
Discrimination is achieved by exploiting the several levels <strong>of</strong> standardized sensitivity:<br />
30 mA, 100 mA, 300 mA and 1 A and the corresponding tripping times, as shown<br />
opposite page in Figure F34.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
10,000<br />
1,000<br />
500<br />
300<br />
250<br />
200<br />
150<br />
130<br />
100<br />
60<br />
40<br />
10<br />
t (ms)<br />
15<br />
30<br />
Fig. F34 : Total discrimination at 2 levels<br />
Fig. F35 : Total discrimination at 2 levels<br />
A<br />
A<br />
60<br />
100<br />
RCD 300 mA<br />
type S<br />
Relay with separate<br />
toroidal CT 3 A<br />
delay time 500 ms<br />
RCD<br />
30 mA<br />
Fig. F36 : Total discrimination at 3 or 4 levels<br />
B<br />
RCCB 1 A<br />
delay time 250 ms<br />
C<br />
150<br />
B<br />
RCCB 300 A<br />
delay time 50 ms<br />
or type S<br />
D<br />
RCCB<br />
30 mA<br />
300<br />
500<br />
600<br />
1,000<br />
5 Implementation <strong>of</strong> the TT system<br />
Discrimination at 2 levels (see Fig. F35)<br />
Protection<br />
b Level A: RCD time-delayed setting I (for industrial device) or type S (for domestic<br />
device) for protection against indirect contacts<br />
b Level B: RCD instantaneous, with high sensitivity on circuits supplying socketoutlets<br />
or appliances at high risk (washing machines, etc.) See also <strong>Chapter</strong> P<br />
Clause 3<br />
Schneider Electric solutions<br />
b Level A: Compact or Multi 9 circuit-breaker with adaptable RCD module<br />
(Vigi NS160 or Vigi NC100), setting I or S type<br />
b Level B: Circuit-breaker with integrated RCD module (DPN Vigi) or adaptable<br />
RCD module (e.g. Vigi C60 or Vigi NC100) or Vigicompact<br />
Note: The setting <strong>of</strong> upstream RCCB must comply with selectivity <strong>rules</strong> and take into<br />
account all the downstream earth leakage currents.<br />
Discrimination at 3 or 4 levels (see Fig. F36)<br />
Protection<br />
b Level A: RCD time-delayed (setting III)<br />
b Level B: RCD time-delayed (setting II)<br />
b Level C: RCD time-delayed (setting I) or type S<br />
b Level D: RCD instantaneous<br />
Schneider Electric solutions<br />
b Level A: Circuit-breaker associated with RCD and separate toroidal transformer<br />
(Vigirex RH54A)<br />
b Level B: Vigicompact or Vigirex<br />
b Level C: Vigirex, Vigicompact or Vigi NC100 or Vigi C60<br />
b Level D:<br />
v Vigicompact or<br />
v Vigirex or<br />
v Multi 9 with integrated or adaptable RCD module : Vigi C60 or DPN Vigi<br />
Note: The setting <strong>of</strong> upstream RCCB must comply with selectivity <strong>rules</strong> and take into<br />
account all the downstream earth leakage currents<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Current<br />
(mA)<br />
1 1.5 10 100 500 1,000 (A)<br />
II<br />
I<br />
selective RCDs<br />
domestic S<br />
and industrial<br />
(settings I and II)<br />
RCD 30 mA<br />
general domestic<br />
and industrial setting 0<br />
F21<br />
© Schneider Electric - all rights reserved
F22<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
MV/LV<br />
NC100<br />
diff.<br />
300 mA<br />
selective<br />
S<br />
5 Implementation <strong>of</strong> the TT system<br />
Discriminative protection at three levels (see Fig. F37)<br />
Withdrawable Masterpact<br />
or Visucompact<br />
NC100L MA<br />
intantaneous<br />
300 mA<br />
Vigicompact<br />
NS100<br />
Setting 1<br />
300 mA<br />
Leakage current<br />
<strong>of</strong> the filter: 20 mA Terminal<br />
board<br />
Fig. F37 : Typical 3-level <strong>installation</strong>, showing the protection <strong>of</strong> distribution circuits in a TT-earthed system. One motor is provided with specific protection<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Discont.<br />
Leakage current equal to 3,5 mA per<br />
socket outlet (Information technology<br />
equipement): max 4 sockets outlets.<br />
NS100 MA
F - Protection against electric shock<br />
Three methods <strong>of</strong> calculation are commonly<br />
used:<br />
b The method <strong>of</strong> impedances, based on the<br />
trigonometric addition <strong>of</strong> the system resistances<br />
and inductive reactances<br />
b The method <strong>of</strong> composition<br />
b The conventional method, based on an<br />
assumed voltage drop and the use <strong>of</strong> prepared<br />
tables<br />
6 Implementation <strong>of</strong> the TN system<br />
6.1 Preliminary conditions<br />
At the <strong>design</strong> stage, the maximum permitted lengths <strong>of</strong> cable downstream <strong>of</strong> a<br />
protective circuit-breaker (or set <strong>of</strong> fuses) must be calculated, while during the<br />
<strong>installation</strong> work certain <strong>rules</strong> must be fully respected.<br />
Certain conditions must be observed, as listed below and illustrated in Figure F38.<br />
1. PE conductor must be regularly connected to earth as much as possible.<br />
2. The PE conductor must not pass through ferro-magnetic conduit, ducts, etc. or<br />
be mounted on steel work, since inductive and/or proximity effects can increase the<br />
effective impedance <strong>of</strong> the conductor.<br />
3. In the case <strong>of</strong> a PEN conductor (a neutral conductor which is also used as a<br />
protective conductor), connection must be made directly to the earth terminal <strong>of</strong> an<br />
appliance (see 3 in Figure F38) before being looped to the neutral terminal <strong>of</strong> the<br />
same appliance.<br />
4. Where the conductor y 6 mm 2 for copper or 10 mm 2 for aluminium, or where a<br />
cable is movable, the neutral and protective conductors should be separated (i.e. a<br />
TN-S system should be adopted within the <strong>installation</strong>).<br />
5. Earth faults may be cleared by overcurrent-protection devices, i.e. by fuses and<br />
circuit-breakers.<br />
The foregoing list indicates the conditions to be respected in the implementation <strong>of</strong> a<br />
TN scheme for the protection against indirect contacts.<br />
RpnA<br />
PEN<br />
Notes:<br />
b The TN scheme requires that the LV neutral <strong>of</strong> the MV/LV transformer, the exposed<br />
conductive parts <strong>of</strong> the substation and <strong>of</strong> the <strong>installation</strong>, and the extraneous conductive<br />
parts in the substation and <strong>installation</strong>, all be earthed to a common earthing system.<br />
b For a substation in which the metering is at low-voltage, a means <strong>of</strong> isolation is required at<br />
the origin <strong>of</strong> the LV <strong>installation</strong>, and the isolation must be clearly visible.<br />
b A PEN conductor must never be interrupted under any circumstances. Control and<br />
protective switchgear for the several TN arrangements will be:<br />
v 3-pole when the circuit includes a PEN conductor,<br />
v Preferably 4-pole (3 phases + neutral) when the circuit includes a neutral with a separate<br />
PE conductor.<br />
Fig. F38 : Implementation <strong>of</strong> the TN system <strong>of</strong> earthing<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
1<br />
5<br />
2 2<br />
3<br />
5<br />
PE N<br />
4<br />
TN-C system TN-C-S system<br />
6.2 Protection against indirect contact<br />
Methods <strong>of</strong> determining levels <strong>of</strong> short-circuit current<br />
In TN-earthed systems, a short-circuit to earth will, in principle, always provide<br />
sufficient current to operate an overcurrent device.<br />
The source and supply mains impedances are much lower than those <strong>of</strong> the<br />
<strong>installation</strong> circuits, so that any restriction in the magnitude <strong>of</strong> earth-fault currents<br />
will be mainly caused by the <strong>installation</strong> conductors (long flexible leads to appliances<br />
greatly increase the “fault-loop” impedance, with a corresponding reduction <strong>of</strong> shortcircuit<br />
current).<br />
The most recent IEC recommendations for indirect-contact protection on TN earthing<br />
systems only relates maximum allowable tripping times to the nominal system<br />
voltage (see Figure F12 in Sub-clause 3.3).<br />
5<br />
F23<br />
© Schneider Electric - all rights reserved
F24<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
For calculations, modern practice is to use<br />
s<strong>of</strong>tware agreed by National Authorities, and<br />
based on the method <strong>of</strong> impedances, such as<br />
Ecodial 3. National Authorities generally also<br />
publish Guides, which include typical values,<br />
conductor lengths, etc.<br />
(1) This results in a calculated current value which is less than<br />
that it would actually flow. If the overcurrent settings are based<br />
on this calculated value, then operation <strong>of</strong> the relay, or fuse, is<br />
assured.<br />
6 Implementation <strong>of</strong> the TN system<br />
The reasoning behind these recommendations is that, for TN systems, the current<br />
which must flow in order to raise the potential <strong>of</strong> an exposed conductive part to 50 V<br />
or more is so high that one <strong>of</strong> two possibilities will occur:<br />
b Either the fault path will blow itself clear, practically instantaneously, or<br />
b The conductor will weld itself into a solid fault and provide adequate current to<br />
operate overcurrent devices<br />
To ensure correct operation <strong>of</strong> overcurrent devices in the latter case, a reasonably<br />
accurate assessment <strong>of</strong> short-circuit earth-fault current levels must be determined at<br />
the <strong>design</strong> stage <strong>of</strong> a project.<br />
A rigorous analysis requires the use <strong>of</strong> phase-sequence-component techniques<br />
applied to every circuit in turn. The principle is straightforward, but the amount <strong>of</strong><br />
computation is not considered justifiable, especially since the zero-phase-sequence<br />
impedances are extremely difficult to determine with any reasonable degree <strong>of</strong><br />
accuracy in an average LV <strong>installation</strong>.<br />
Other simpler methods <strong>of</strong> adequate accuracy are preferred. Three practical methods<br />
are:<br />
b The “method <strong>of</strong> impedances”, based on the summation <strong>of</strong> all the impedances<br />
(positive-phase-sequence only) around the fault loop, for each circuit<br />
b The “method <strong>of</strong> composition”, which is an estimation <strong>of</strong> short-circuit current at<br />
the remote end <strong>of</strong> a loop, when the short-circuit current level at the near end <strong>of</strong> the<br />
loop is known<br />
b The “conventional method” <strong>of</strong> calculating the minimum levels <strong>of</strong> earth-fault<br />
currents, together with the use <strong>of</strong> tables <strong>of</strong> values for obtaining rapid results<br />
These methods are only reliable for the case in which the cables that make up the<br />
earth-fault-current loop are in close proximity (to each other) and not separated by<br />
ferro-magnetic materials.<br />
Method <strong>of</strong> impedances<br />
This method summates the positive-sequence impedances <strong>of</strong> each item (cable, PE<br />
conductor, transformer, etc.) included in the earth-fault loop circuit from which the<br />
short-circuit earth-fault current is calculated, using the formula:<br />
U<br />
I =<br />
2 2<br />
( R) + ( X)<br />
� �<br />
where<br />
(ΣR) 2 = (the sum <strong>of</strong> all resistances in the loop) 2 at the <strong>design</strong> stage <strong>of</strong> a project.<br />
and (ΣX) 2 = (the sum <strong>of</strong> all inductive reactances in the loop) 2<br />
and U = nominal system phase-to-neutral voltage.<br />
The application <strong>of</strong> the method is not always easy, because it supposes a knowledge<br />
<strong>of</strong> all parameter values and characteristics <strong>of</strong> the elements in the loop. In many<br />
cases, a national guide can supply typical values for estimation purposes.<br />
Method <strong>of</strong> composition<br />
This method permits the determination <strong>of</strong> the short-circuit current at the end <strong>of</strong><br />
a loop from the known value <strong>of</strong> short-circuit at the sending end, by means <strong>of</strong> the<br />
approximate formula:<br />
U<br />
I = Isc<br />
U+ Zs. Isc<br />
where<br />
Isc where = upstream short-circuit current<br />
I = end-<strong>of</strong>-loop short-circuit current<br />
U = nominal system phase voltage<br />
Zs = impedance <strong>of</strong> loop<br />
Note: in this method the individual impedances are added arithmetically (1) as<br />
opposed to the previous “method <strong>of</strong> impedances” procedure.<br />
Conventional method<br />
This method is generally considered to be sufficiently accurate to fix the upper limit<br />
<strong>of</strong> cable lengths.<br />
Principle<br />
The principle bases the short-circuit current calculation on the assumption that the<br />
voltage at the origin <strong>of</strong> the circuit concerned (i.e. at the point at which the circuit<br />
protective device is located) remains at 80% or more <strong>of</strong> the nominal phase to neutral<br />
voltage. The 80% value is used, together with the circuit loop impedance, to compute<br />
the short-circuit current.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
The maximum length <strong>of</strong> any circuit <strong>of</strong> a<br />
TN-earthed <strong>installation</strong> is: 0.8 Uo Sph<br />
max =<br />
� 1+ m Ia<br />
( )<br />
The following tables give the length <strong>of</strong> circuit<br />
which must not be exceeded, in order that<br />
persons be protected against indirect contact<br />
hazards by protective devices<br />
Id<br />
A B<br />
SPE Sph<br />
C<br />
Imagn<br />
Fig. F39 : Calculation <strong>of</strong> L max. for a TN-earthed system, using<br />
the conventional method<br />
L<br />
PE<br />
(1) For the definition <strong>of</strong> type B, C, D circuit-breakers, refer to<br />
chapter H, clause 4.2<br />
6 Implementation <strong>of</strong> the TN system<br />
This coefficient takes account <strong>of</strong> all voltage drops upstream <strong>of</strong> the point considered.<br />
In LV cables, when all conductors <strong>of</strong> a 3-phase 4-wire circuit are in close proximity<br />
(which is the normal case), the inductive reactance internal to and between<br />
conductors is negligibly small compared to the cable resistance.<br />
This approximation is considered to be valid for cable sizes up to 120 mm 2 .<br />
Above that size, the resistance value R is increased as follows:<br />
Core size (mm 2 ) Value <strong>of</strong> resistance<br />
S = 150 mm 2 R+15%<br />
S = 185 mm 2 R+20%<br />
S = 240 mm 2 R+25%<br />
The maximum length <strong>of</strong> a circuit in a TN-earthed <strong>installation</strong> is given by the formula:<br />
0.8 Uo Sph<br />
Lmax<br />
=<br />
� 1+ m Ia<br />
( )<br />
where:<br />
Lmax = maximum length in metres<br />
Uo = phase volts = 230 V for a 230/400 V system<br />
ρ = resistivity at normal working temperature in ohm-mm2 /metre<br />
(= 22.5 10-3 for copper; = 36 10-3 for aluminium)<br />
Ia = trip current setting for the instantaneous operation <strong>of</strong> a circuit-breaker, or<br />
Ia = the current which assures operation <strong>of</strong> the protective fuse concerned, in the<br />
specified time.<br />
m Sph<br />
=<br />
SPE<br />
Sph = cross-sectional area <strong>of</strong> the phase conductors <strong>of</strong> the circuit concerned in mm 2<br />
SPE = cross-sectional area <strong>of</strong> the protective conductor concerned in mm 2 .<br />
(see Fig. F39)<br />
Tables<br />
The following tables, applicable to TN systems, have been established according to<br />
the “conventional method” described above.<br />
The tables give maximum circuit lengths, beyond which the ohmic resistance <strong>of</strong> the<br />
conductors will limit the magnitude <strong>of</strong> the short-circuit current to a level below that<br />
required to trip the circuit-breaker (or to blow the fuse) protecting the circuit, with<br />
sufficient rapidity to ensure safety against indirect contact.<br />
Correction factor m<br />
Figure F40 indicates the correction factor to apply to the values given in Figures F41<br />
to F44 next pages, according to the ratio Sph/SPE, the type <strong>of</strong> circuit, and the<br />
conductor materials.<br />
The tables take into account:<br />
b The type <strong>of</strong> protection: circuit-breakers or fuses<br />
b Operating-current settings<br />
b Cross-sectional area <strong>of</strong> phase conductors and protective conductors<br />
b Type <strong>of</strong> system earthing (see Fig. F45 page F27)<br />
b Type <strong>of</strong> circuit-breaker (i.e. B, C or D) (1)<br />
The tables may be used for 230/400 V systems.<br />
Equivalent tables for protection by Compact and Multi 9 circuit-breakers (Merlin<br />
Gerin) are included in the relevant catalogues.<br />
Circuit Conductor m = Sph/SPE (or PEN)<br />
material m = 1 m = 2 m = 3 m = 4<br />
3P + N or P + N Copper 1 0.67 0.50 0.40<br />
Aluminium 0.62 0.42 0.31 0.25<br />
Fig. F40 : Correction factor to apply to the lengths given in tables F40 to F43 for<br />
TN systems<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
F25<br />
© Schneider Electric - all rights reserved
F26<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
Circuits protected by general purpose circuit-breakers (Fig. F41)<br />
Nominal Instantaneous or short-time-delayed tripping current Im (amperes)<br />
cross-<br />
sectional<br />
area<br />
<strong>of</strong><br />
conductors<br />
mm 2 50 63 80 100 125 160 200 250 320 400 500 560 630 700 800 875 1000 1120 1250 1600 2000 2500 3200 4000 5000 6300 8000 10000 12500<br />
1.5 100 79 63 50 40 31 25 20 16 13 10 9 8 7 6 6 5 4 4<br />
2.5 167 133 104 83 67 52 42 33 26 21 17 15 13 12 10 10 8 7 7 5 4<br />
4 267 212 167 133 107 83 67 53 42 33 27 24 21 19 17 15 13 12 11 8 7 5 4<br />
6 400 317 250 200 160 125 100 80 63 50 40 36 32 29 25 23 20 18 16 13 10 8 6 5 4<br />
10 417 333 267 208 167 133 104 83 67 60 53 48 42 38 33 30 27 21 17 13 10 8 7 5 4<br />
16 427 333 267 213 167 133 107 95 85 76 67 61 53 48 43 33 27 21 17 13 11 8 7 5 4<br />
25 417 333 260 208 167 149 132 119 104 95 83 74 67 52 42 33 26 21 17 13 10 8 7<br />
35 467 365 292 233 208 185 167 146 133 117 104 93 73 58 47 36 29 23 19 15 12 9<br />
50 495 396 317 283 251 226 198 181 158 141 127 99 79 63 49 40 32 25 20 16 13<br />
70 417 370 333 292 267 233 208 187 146 117 93 73 58 47 37 29 23 19<br />
95 452 396 362 317 283 263 198 158 127 99 79 63 50 40 32 25<br />
120 457 400 357 320 250 200 160 125 100 80 63 50 40 32<br />
150 435 388 348 272 217 174 136 109 87 69 54 43 35<br />
185 459 411 321 257 206 161 128 103 82 64 51 41<br />
240 400 320 256 200 160 128 102 80 64 51<br />
Fig. F41 : Maximum circuit lengths (in metres) for different sizes <strong>of</strong> copper conductor and instantaneous-tripping-current settings for general-purpose circuit-breakers<br />
in 230/240 V TN system with m = 1<br />
Circuits protected by Compact (1) or Multi 9 (1) circuit-breakers for industrial or<br />
domestic use (Fig. F42 to Fig. F44)<br />
Sph Rated current (A)<br />
mm 2 1 2 3 4 6 10 16 20 25 32 40 50 63 80 100 125<br />
1.5 1200 600 400 300 200 120 75 60 48 37 30 24 19 15 12 10<br />
2.5 1000 666 500 333 200 125 100 80 62 50 40 32 25 20 16<br />
4 1066 800 533 320 200 160 128 100 80 64 51 40 32 26<br />
6 1200 800 480 300 240 192 150 120 96 76 60 48 38<br />
10 800 500 400 320 250 200 160 127 100 80 64<br />
16 800 640 512 400 320 256 203 160 128 102<br />
25 800 625 500 400 317 250 200 160<br />
35 875 700 560 444 350 280 224<br />
50 760 603 475 380 304<br />
Fig. F42 : Maximum circuit lengths (in meters) for different sizes <strong>of</strong> copper conductor and rated currents for type B (2) circuit-breakers in a 230/240 V single-phase or<br />
three-phase TN system with m = 1<br />
(1) Merlin Gerin products.<br />
(2) For the definition <strong>of</strong> type B and C circuit-breakers refer to<br />
chapter H clause 4.2.<br />
6 Implementation <strong>of</strong> the TN system<br />
Sph Rated current (A)<br />
mm 2 1 2 3 4 6 10 16 20 25 32 40 50 63 80 100 125<br />
1.5 600 300 200 150 100 60 37 30 24 18 15 12 9 7 6 5<br />
2.5 500 333 250 167 100 62 50 40 31 25 20 16 12 10 8<br />
4 533 400 267 160 100 80 64 50 40 32 25 20 16 13<br />
6 600 400 240 150 120 96 75 60 48 38 30 24 19<br />
10 667 400 250 200 160 125 100 80 63 50 40 32<br />
16 640 400 320 256 200 160 128 101 80 64 51<br />
25 625 500 400 312 250 200 159 125 100 80<br />
35 875 700 560 437 350 280 222 175 140 112<br />
50 760 594 475 380 301 237 190 152<br />
Fig. F43 : Maximum circuit lengths (in metres) for different sizes <strong>of</strong> copper conductor and rated currents for type C (2) circuit-breakers in a 230/240 V single-phase or<br />
three-phase TN system with m = 1<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
6 Implementation <strong>of</strong> the TN system<br />
Sph Rated current (A)<br />
mm 2 1 2 3 4 6 10 16 20 25 32 40 50 63 80 100 125<br />
1.5 429 214 143 107 71 43 27 21 17 13 11 9 7 5 4 3<br />
2.5 714 357 238 179 119 71 45 36 29 22 18 14 11 9 7 6<br />
4 571 381 286 190 114 71 80 46 36 29 23 18 14 11 9<br />
6 857 571 429 286 171 107 120 69 54 43 34 27 21 17 14<br />
10 952 714 476 286 179 200 114 89 71 57 45 36 29 23<br />
16 762 457 286 320 183 143 114 91 73 57 46 37<br />
25 714 446 500 286 223 179 143 113 89 71 57<br />
35 625 700 400 313 250 200 159 125 80 100<br />
50 848 543 424 339 271 215 170 136 109<br />
Fig. F44 : Maximum circuit lengths (in metres) for different sizes <strong>of</strong> copper conductor and rated currents for type D (1) circuit-breakers in a 230/240 V single-phase or<br />
three-phase TN system with m = 1<br />
Fig. F45 : Separate earth electrode<br />
RA1 RA2<br />
Fig. F46 : Circuit supplying socket-outlets<br />
Distant location<br />
(1) For the definition <strong>of</strong> type D circuit-breaker refer to chapter H<br />
Sub-clause 4.2.<br />
Example<br />
A 3-phase 4-wire (230/400 V) <strong>installation</strong> is TN-C earthed. A circuit is protected by a<br />
type B circuit-breaker rated at 63 A, and consists <strong>of</strong> an aluminium cored cable with<br />
50 mm 2 phase conductors and a neutral conductor (PEN) <strong>of</strong> 25 mm 2 .<br />
What is the maximum length <strong>of</strong> circuit, below which protection <strong>of</strong> persons against<br />
indirect-contact hazards is assured by the instantaneous magnetic tripping relay <strong>of</strong><br />
the circuit-breaker?<br />
Figure F42 gives, for 50 mm2 and a 63 A type B circuit-breaker, 603 metres, to which<br />
must be applied a factor <strong>of</strong> 0.42 (Figure F40 for m Sph<br />
= = 2).<br />
SPE<br />
The maximum length <strong>of</strong> circuit is therefore:<br />
603 x 0.42 = 253 metres.<br />
Particular case where one or more exposed conductive part(s)<br />
is (are) earthed to a separate earth electrode<br />
Protection must be provided against indirect contact by a RCD at the origin <strong>of</strong> any<br />
circuit supplying an appliance or group <strong>of</strong> appliances, the exposed conductive parts<br />
<strong>of</strong> which are connected to an independent earth electrode.<br />
The sensitivity <strong>of</strong> the RCD must be adapted to the earth electrode resistance (RA2 in<br />
Figure F45). See specifications applicable to TT system.<br />
6.3 High-sensitivity RCDs (see Fig. F31)<br />
According to IEC 60364-4-41, high sensitivity RCDs (y 30 mA) must be used for<br />
protection <strong>of</strong> socket outlets with rated current y 20 A in all locations. The use <strong>of</strong> such<br />
RCDs is also recommended in the following cases:<br />
b Socket-outlet circuits in wet locations at all current ratings<br />
b Socket-outlet circuits in temporary <strong>installation</strong>s<br />
b Circuits supplying laundry rooms and swimming pools<br />
b Supply circuits to work-sites, caravans, pleasure boats, and travelling fairs<br />
See 2.2 and chapter P, al section 3.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
F27<br />
© Schneider Electric - all rights reserved
F28<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
Fig. F47 : Fire-risk location<br />
2 y Irm y 4In<br />
Great length <strong>of</strong> cable<br />
Fire-risk<br />
location<br />
PE or PEN<br />
Fig. F48 : Circuit-breaker with low-set instantaneous magnetic<br />
tripping<br />
Phases<br />
Neutral<br />
PE<br />
6.4 Protection in high fire-risk location<br />
According to IEC 60364-422-3.10, circuits in high fire-risk locations must be<br />
protected by RCDs <strong>of</strong> sensitivity y 500 mA. This excludes the TN-C arrangement and<br />
TN-S must be adopted.<br />
A preferred sensitivity <strong>of</strong> 300 mA is mandatory in some countries (see Fig. F47).<br />
6.5 When the fault current-loop impedance is<br />
particularly high<br />
When the earth-fault current is limited due to an inevitably high fault-loop impedance,<br />
so that the overcurrent protection cannot be relied upon to trip the circuit within the<br />
prescribed time, the following possibilities should be considered:<br />
Suggestion 1 (see Fig. F48)<br />
b Install a circuit-breaker which has a lower instantaneous magnetic tripping level, for<br />
example:<br />
2In y Irm y 4In<br />
This affords protection for persons on circuits which are abnormally long. It must<br />
be checked, however, that high transient currents such as the starting currents <strong>of</strong><br />
motors will not cause nuisance trip-outs.<br />
b Schneider Electric solutions<br />
v Type G Compact (2Im y Irm y 4Im)<br />
v Type B Multi 9 circuit-breaker<br />
Suggestion 2 (see Fig. F49)<br />
b Install a RCD on the circuit. The device does not need to be highly-sensitive<br />
(HS) (several amps to a few tens <strong>of</strong> amps). Where socket-outlets are involved, the<br />
particular circuits must, in any case, be protected by HS (y 30 mA) RCDs; generally<br />
one RCD for a number <strong>of</strong> socket outlets on a common circuit.<br />
b Schneider Electric solutions<br />
v RCD Multi 9 NG125 : IΔn = 1 or 3 A<br />
v Vigicompact REH or REM: IΔn = 3 to 30 A<br />
v Type B Multi 9 circuit-breaker<br />
Fig. F49 : RCD protection on TN systems with high earth-faultloop<br />
impedance Fig. F50 : Improved equipotential bonding<br />
6 Implementation <strong>of</strong> the TN system<br />
Suggestion 3<br />
Increase the size <strong>of</strong> the PE or PEN conductors and/or the phase conductors, to<br />
reduce the loop impedance.<br />
Suggestion 4<br />
Add supplementary equipotential conductors. This will have a similar effect to that<br />
<strong>of</strong> suggestion 3, i.e. a reduction in the earth-fault-loop resistance, while at the same<br />
time improving the existing touch-voltage protection measures. The effectiveness<br />
<strong>of</strong> this improvement may be checked by a resistance test between each exposed<br />
conductive part and the local main protective conductor.<br />
For TN-C <strong>installation</strong>s, bonding as shown in Figure F50 is not allowed, and<br />
suggestion 3 should be adopted.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
(1) On systems where the neutral is distributed, as shown in<br />
Figure F56.<br />
7 Implementation <strong>of</strong> the IT system<br />
The basic feature <strong>of</strong> the IT earthing system is that, in the event <strong>of</strong> a short-circuit to<br />
earth fault, the system can continue to operate without interruption. Such a fault is<br />
referred to as a “first fault”.<br />
In this system, all exposed conductive parts <strong>of</strong> an <strong>installation</strong> are connected via<br />
PE conductors to an earth electrode at the <strong>installation</strong>, while the neutral point <strong>of</strong> the<br />
supply transformer is:<br />
b Either isolated from earth<br />
b Or connected to earth through a high resistance (commonly 1,000 ohms or more)<br />
This means that the current through an earth fault will be measured in milli-amps,<br />
which will not cause serious damage at the fault position, or give rise to dangerous<br />
touch voltages, or present a fire hazard. The system may therefore be allowed to<br />
operate normally until it is convenient to isolate the faulty section for repair work. This<br />
enhances continuity <strong>of</strong> service.<br />
In practice, the system earthing requires certain specific measures for its satisfactory<br />
exploitation:<br />
b Permanent monitoring <strong>of</strong> the insulation with respect to earth, which must signal<br />
(audibly or visually) the occurrence <strong>of</strong> the first fault<br />
b A device for limiting the voltage which the neutral point <strong>of</strong> the supply transformer<br />
can reach with respect to earth<br />
b A “first-fault” location routine by an efficient maintenance staff. Fault location is<br />
greatly facilitated by automatic devices which are currently available<br />
b Automatic high-speed tripping <strong>of</strong> appropriate circuit-breakers must take place in<br />
the event <strong>of</strong> a “second fault” occurring before the first fault is repaired. The second<br />
fault (by definition) is an earth fault affecting a different live conductor than that <strong>of</strong> the<br />
first fault (can be a phase or neutral conductor) (1) .<br />
The second fault results in a short-circuit through the earth and/or through<br />
PE bonding conductors.<br />
7.1 Preliminary conditions (see Fig. F51 and Fig. F52)<br />
Minimum functions required Components and devices Examples<br />
Protection against overvoltages (1) Voltage limiter Cardew C<br />
at power frequency<br />
Neutral earthing resistor (2) Resistor Impedance Zx<br />
(for impedance earthing variation)<br />
Overall earth-fault monitor (3) Permanent insulation Vigilohm TR22A<br />
with alarm for first fault condition monitor PIM with alarm feature or XM 200<br />
Automatic fault clearance (4) Four-pole circuit-breakers Compact circuit-breaker<br />
on second fault and (if the neutral is distributed) or RCD-MS<br />
protection <strong>of</strong> the neutral all 4 poles trip<br />
conductor against overcurrent<br />
Location <strong>of</strong> first fault (5) With device for fault-location Vigilohm system<br />
on live system, or by successive<br />
opening <strong>of</strong> circuits<br />
Fig. F51 : Essential functions in IT schemes and examples with Merlin Gerin products<br />
HV/LV<br />
2 1 3<br />
Fig. F52 : Positions <strong>of</strong> essential functions in 3-phase 3-wire IT-earthed system<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
4<br />
4<br />
5<br />
4<br />
L1<br />
L2<br />
L3 N<br />
F29<br />
© Schneider Electric - all rights reserved
F30<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
Modern monitoring systems greatly facilitate<br />
first-fault location and repair<br />
Fault-location systems comply with<br />
IEC 61157-9 standard<br />
MERLIN GERIN<br />
XM100<br />
XM100<br />
(1) On a 230/400 V 3-phase system.<br />
GR10X<br />
Fig. F53 : Non-automatic (manual) fault location<br />
7 Implementation <strong>of</strong> the IT system<br />
7.2 Protection against indirect contact<br />
First-fault condition<br />
The earth-fault current which flows under a first-fault condition is measured in milliamps.<br />
The fault voltage with respect to earth is the product <strong>of</strong> this current and the<br />
resistance <strong>of</strong> the <strong>installation</strong> earth electrode and PE conductor (from the faulted<br />
component to the electrode). This value <strong>of</strong> voltage is clearly harmless and could<br />
amount to several volts only in the worst case (1,000 Ω earthing resistor will pass<br />
230 mA (1) and a poor <strong>installation</strong> earth-electrode <strong>of</strong> 50 ohms, would give 11.5 V, for<br />
example).<br />
An alarm is given by the permanent insulation monitoring device.<br />
Principle <strong>of</strong> earth-fault monitoring<br />
A generator <strong>of</strong> very low frequency a.c. current, or <strong>of</strong> d.c. current, (to reduce the<br />
effects <strong>of</strong> cable capacitance to negligible levels) applies a voltage between the<br />
neutral point <strong>of</strong> the supply transformer and earth. This voltage causes a small current<br />
to flow according to the insulation resistance to earth <strong>of</strong> the whole <strong>installation</strong>, plus<br />
that <strong>of</strong> any connected appliance.<br />
Low-frequency instruments can be used on a.c. systems which generate transient<br />
d.c. components under fault conditions. Certain versions can distinguish between<br />
resistive and capacitive components <strong>of</strong> the leakage current.<br />
Modern equipment allow the measurement <strong>of</strong> leakage-current evolution, so that<br />
prevention <strong>of</strong> a first fault can be achieved.<br />
Examples <strong>of</strong> equipment<br />
b Manual fault-location (see Fig. F53)<br />
The generator may be fixed (example: XM100) or portable (example: GR10X<br />
permitting the checking <strong>of</strong> dead circuits) and the receiver, together with the magnetic<br />
clamp-type pick-up sensor, are portable.<br />
RM10N<br />
P12 P50<br />
P100 ON/OFF<br />
b Fixed automatic fault location (see Fig. F54 next page)<br />
The monitoring relay XM100, together with the fixed detectors XD1 or XD12 (each<br />
connected to a toroidal CT embracing the conductors <strong>of</strong> the circuit concerned)<br />
provide a system <strong>of</strong> automatic fault location on a live <strong>installation</strong>.<br />
Moreover, the level <strong>of</strong> insulation is indicated for each monitored circuit, and two<br />
levels are checked: the first level warns <strong>of</strong> unusually low insulation resistance so that<br />
preventive measures may be taken, while the second level indicates a fault condition<br />
and gives an alarm.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
MERLIN GERIN<br />
XM100<br />
XM100<br />
Fig. F54 : Fixed automatic fault location<br />
MERLIN GERIN<br />
XM100<br />
XD1<br />
XM100<br />
Fig. F55 : Automatic fault location and insulation-resistance data logging<br />
7 Implementation <strong>of</strong> the IT system<br />
XD1 XD1 XD12<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Toroidal CTs<br />
1 to 12 circuits<br />
b Automatic monitoring, logging, and fault location (see Fig. F55)<br />
The Vigilohm System also allows access to a printer and/or a PC which provides<br />
a global review <strong>of</strong> the insulation level <strong>of</strong> an entire <strong>installation</strong>, and records the<br />
chronological evolution <strong>of</strong> the insulation level <strong>of</strong> each circuit.<br />
The central monitor XM100, together with the localization detectors XD08 and XD16,<br />
associated with toroidal CTs from several circuits, as shown below in Figure F55,<br />
provide the means for this automatic exploitation.<br />
MERLIN GERIN<br />
XL08<br />
897<br />
MERLIN GERIN<br />
XD08 XD16<br />
XL16<br />
678<br />
F31<br />
© Schneider Electric - all rights reserved
F32<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
Three methods <strong>of</strong> calculation are commonly<br />
used:<br />
b The method <strong>of</strong> impedances, based on the<br />
trigonometric addition <strong>of</strong> the system resistances<br />
and inductive reactances<br />
b The method <strong>of</strong> composition<br />
b The conventional method, based on an<br />
assumed voltage drop and the use <strong>of</strong> prepared<br />
tables<br />
7 Implementation <strong>of</strong> the IT system<br />
Implementation <strong>of</strong> permanent insulation-monitoring (PIM) devices<br />
b Connection<br />
The PIM device is normally connected between the neutral (or articificial neutral)<br />
point <strong>of</strong> the power-supply transformer and its earth electrode.<br />
b Supply<br />
Power supply to the PIM device should be taken from a highly reliable source. In<br />
practice, this is generally directly from the <strong>installation</strong> being monitored, through<br />
overcurrent protective devices <strong>of</strong> suitable short-circuit current rating.<br />
b Level settings<br />
Certain national standards recommend a first setting at 20% below the insulation<br />
level <strong>of</strong> the new <strong>installation</strong>. This value allows the detection <strong>of</strong> a reduction <strong>of</strong> the<br />
insulation quality, necessitating preventive maintenance measures in a situation <strong>of</strong><br />
incipient failure.<br />
The detection level for earth-fault alarm will be set at a much lower level.<br />
By way <strong>of</strong> an example, the two levels might be:<br />
v New <strong>installation</strong> insulation level: 100 kΩ<br />
v Leakage current without danger: 500 mA (fire risk at > 500 mA)<br />
v Indication levels set by the consumer:<br />
- Threshold for preventive maintenance: 0.8 x 100 = 80 kΩ<br />
- Threshold for short-circuit alarm: 500 Ω<br />
Notes:<br />
v Following a long period <strong>of</strong> shutdown, during which the whole, or part <strong>of</strong> the <strong>installation</strong><br />
remains de-energized, humidity can reduce the general level <strong>of</strong> insulation resistance.<br />
This situation, which is mainly due to leakage current over the damp surface <strong>of</strong><br />
healthy insulation, does not constitute a fault condition, and will improve rapidly as the<br />
normal temperature rise <strong>of</strong> current-carrying conductors reduces the surface humidity.<br />
v The PIM device (XM) can measure separately the resistive and the capacitive<br />
current components <strong>of</strong> the leakage current to earth, thereby deriving the true<br />
insulation resistance from the total permanent leakage current.<br />
The case <strong>of</strong> a second fault<br />
A second earth fault on an IT system (unless occurring on the same conductor<br />
as the first fault) constitutes a phase-phase or phase-to-neutral fault, and whether<br />
occurring on the same circuit as the first fault, or on a different circuit, overcurrent<br />
protective devices (fuses or circuit-breakers) would normally operate an automatic<br />
fault clearance.<br />
The settings <strong>of</strong> overcurrent tripping relays and the ratings <strong>of</strong> fuses are the basic<br />
parameters that decide the maximum practical length <strong>of</strong> circuit that can be<br />
satisfactorily protected, as discussed in Sub-clause 6.2.<br />
Note: In normal circumstances, the fault current path is through common<br />
PE conductors, bonding all exposed conductive parts <strong>of</strong> an <strong>installation</strong>, and so the<br />
fault loop impedance is sufficiently low to ensure an adequate level <strong>of</strong> fault current.<br />
Where circuit lengths are unavoidably long, and especially if the appliances <strong>of</strong> a<br />
circuit are earthed separately (so that the fault current passes through two earth<br />
electrodes), reliable tripping on overcurrent may not be possible.<br />
In this case, an RCD is recommended on each circuit <strong>of</strong> the <strong>installation</strong>.<br />
Where an IT system is resistance earthed, however, care must be taken to ensure<br />
that the RCD is not too sensitive, or a first fault may cause an unwanted trip-out.<br />
Tripping <strong>of</strong> residual current devices which satisfy IEC standards may occur at values<br />
<strong>of</strong> 0.5 ΙΔn to ΙΔn, where ΙΔn is the nominal residual-current setting level.<br />
Methods <strong>of</strong> determining levels <strong>of</strong> short-circuit current<br />
A reasonably accurate assessment <strong>of</strong> short-circuit current levels must be carried out<br />
at the <strong>design</strong> stage <strong>of</strong> a project.<br />
A rigorous analysis is not necessary, since current magnitudes only are important for<br />
the protective devices concerned (i.e. phase angles need not be determined) so that<br />
simplified conservatively approximate methods are normally used. Three practical<br />
methods are:<br />
b The method <strong>of</strong> impedances, based on the vectorial summation <strong>of</strong> all the (positivephase-sequence)<br />
impedances around a fault-current loop<br />
b The method <strong>of</strong> composition, which is an approximate estimation <strong>of</strong> short-circuit<br />
current at the remote end <strong>of</strong> a loop, when the level <strong>of</strong> short-circuit current at the near<br />
end <strong>of</strong> the loop is known. Complex impedances are combined arithmetically in this<br />
method<br />
b The conventional method, in which the minimum value <strong>of</strong> voltage at the origin <strong>of</strong><br />
a faulty circuit is assumed to be 80% <strong>of</strong> the nominal circuit voltage, and tables are<br />
used based on this assumption, to give direct readings <strong>of</strong> circuit lengths.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
The s<strong>of</strong>tware Ecodial is based on the “method<br />
<strong>of</strong> impedance”<br />
The maximum length <strong>of</strong> an IT earthed circuit is:<br />
b For a 3-phase 3-wire scheme<br />
0.8 Uo 3 Sph<br />
Lmax<br />
=<br />
2 �I a 1+<br />
m<br />
( )<br />
b For a 3-phase 4-wire scheme<br />
0.8 Uo S1<br />
Lmax<br />
=<br />
2 �I a 1+<br />
m<br />
( )<br />
N<br />
PE<br />
Id<br />
C<br />
D<br />
Non distributed neutral<br />
(1) Reminder: There is no length limit for earth-fault protection<br />
on a TT scheme, since protection is provided by RCDs <strong>of</strong> high<br />
sensitivity.<br />
7 Implementation <strong>of</strong> the IT system<br />
These methods are reliable only for the cases in which wiring and cables which<br />
make up the fault-current loop are in close proximity (to each other) and are not<br />
separated by ferro-magnetic materials.<br />
Methods <strong>of</strong> impedances<br />
This method as described in Sub-clause 6.2, is identical for both the IT and<br />
TN systems <strong>of</strong> earthing.<br />
Methods <strong>of</strong> composition<br />
This method as described in Sub-clause 6.2, is identical for both the IT and<br />
TN systems <strong>of</strong> earthing.<br />
Conventional method (see Fig. F56)<br />
The principle is the same for an IT system as that described in Sub-clause 6.2 for a<br />
TN system : the calculation <strong>of</strong> maximum circuit lengths which should not be exceeded<br />
downstream <strong>of</strong> a circuit-breaker or fuses, to ensure protection by overcurrent devices.<br />
It is clearly impossible to check circuit lengths for every feasible combination <strong>of</strong> two<br />
concurrent faults.<br />
All cases are covered, however, if the overcurrent trip setting is based on the<br />
assumption that a first fault occurs at the remote end <strong>of</strong> the circuit concerned,<br />
while the second fault occurs at the remote end <strong>of</strong> an identical circuit, as already<br />
mentioned in Sub-clause 3.4. This may result, in general, in one trip-out only<br />
occurring (on the circuit with the lower trip-setting level), thereby leaving the system<br />
in a first-fault situation, but with one faulty circuit switched out <strong>of</strong> service.<br />
b For the case <strong>of</strong> a 3-phase 3-wire <strong>installation</strong> the second fault can only cause a<br />
phase/phase short-circuit, so that the voltage to use in the formula for maximum<br />
circuit length is 3 Uo.<br />
The maximum circuit length is given by:<br />
0.8 Uo 3 Sph<br />
Lmax<br />
=<br />
metres<br />
2 �I a 1+<br />
m<br />
( )<br />
b For the case <strong>of</strong> a 3-phase 4-wire <strong>installation</strong> the lowest value <strong>of</strong> fault current will<br />
occur if one <strong>of</strong> the faults is on a neutral conductor. In this case, Uo is the value to use<br />
for computing the maximum cable length, and<br />
0.8 Uo S1<br />
Lmax<br />
=<br />
2 �I a 1+<br />
m<br />
( )<br />
metres<br />
i.e. 50% only <strong>of</strong> the length permitted for a TN scheme (1)<br />
Fig. F56 : Calculation <strong>of</strong> Lmax. for an IT-earthed system, showing fault-current path for a double-fault condition<br />
Id<br />
A<br />
B<br />
N<br />
PE<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Id<br />
Id<br />
Distributed neutral<br />
F33<br />
© Schneider Electric - all rights reserved
F34<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
The following tables (1) give the length <strong>of</strong> circuit<br />
which must not be exceeded, in order that<br />
persons be protected against indirect contact<br />
hazards by protective devices<br />
Fig. F62 : Circuit supplying socket-outlets<br />
(1) The tables are those shown in Sub-clause 6.2 (Figures<br />
F41 to F44). However, the table <strong>of</strong> correction factors (Figure<br />
F57) which takes into account the ratio Sph/SPE, and <strong>of</strong> the<br />
type <strong>of</strong> circuit (3-ph 3-wire; 3-ph 4-wire; 1-ph 2-wire) as well<br />
as conductor material, is specific to the IT system, and differs<br />
from that for TN.<br />
7 Implementation <strong>of</strong> the IT system<br />
In the preceding formulae:<br />
Lmax = longest circuit in metres<br />
Uo = phase-to-neutral voltage (230 V on a 230/400 V system)<br />
ρ = resistivity at normal operating temperature (22.5 x 10-3 ohms-mm2 /m for copper,<br />
36 x 10-3 ohms-mm2 /m for aluminium)<br />
Ia = overcurrent trip-setting level in amps, or Ia = current in amps required to clear<br />
the fuse in the specified time<br />
m Sph<br />
=<br />
SPE<br />
SPE = cross-sectional area <strong>of</strong> PE conductor in mm 2<br />
S1 = S neutral if the circuit includes a neutral conductor<br />
S1 = Sph if the circuit does not include a neutral conductor<br />
Tables<br />
The following tables have been established according to the “conventional method”<br />
described above.<br />
The tables give maximum circuit lengths, beyond which the ohmic resistance <strong>of</strong><br />
the conductors will limit the magnitude <strong>of</strong> the short-circuit current to a level below<br />
that required to trip the circuit-breaker (or to blow the fuse) protecting the circuit,<br />
with sufficient rapidity to ensure safety against indirect contact. The tables take into<br />
account:<br />
b The type <strong>of</strong> protection: circuit-breakers or fuses, operating-current settings<br />
b Cross-sectional area <strong>of</strong> phase conductors and protective conductors<br />
b Type <strong>of</strong> earthing scheme<br />
b Correction factor: Figure F57 indicates the correction factor to apply to the lengths<br />
given in tables F40 to F43, when considering an IT system<br />
Circuit Conductor m = Sph/SPE (or PEN)<br />
material m = 1 m = 2 m = 3 m = 4<br />
3 phases Copper 0.86 0.57 0.43 0.34<br />
Aluminium 0.54 0.36 0.27 0.21<br />
3ph + N or 1ph + N Copper 0.50 0.33 0.25 0.20<br />
Aluminium 0.31 0.21 0.16 0.12<br />
Fig. F57 : Correction factor to apply to the lengths given in tables F41 to F44 for TN systems<br />
Example<br />
A 3-phase 3-wire 230/400 V <strong>installation</strong> is IT-earthed.<br />
One <strong>of</strong> its circuits is protected by a circuit-breaker rated at 63 A, and consists <strong>of</strong> an<br />
aluminium-cored cable with 50 mm 2 phase conductors. The 25 mm 2 PE conductor<br />
is also aluminum. What is the maximum length <strong>of</strong> circuit, below which protection <strong>of</strong><br />
persons against indirect-contact hazards is assured by the instantaneous magnetic<br />
tripping relay <strong>of</strong> the circuit-breaker?<br />
Figure F42 indicates 603 metres, to which must be applied a correction factor <strong>of</strong> 0.36<br />
(m = 2 for aluminium cable).<br />
The maximum length is therefore 217 metres.<br />
7.3 High-sensitivity RCDs<br />
According to IEC 60364-4-41, high sensitivity RCDs (y 30 mA) must be used for<br />
protection <strong>of</strong> socket outlets with rated current y 20 A in all locations. The use <strong>of</strong> such<br />
RCDs is also recommended in the following cases:<br />
b Socket-outlet circuits in wet locations at all current ratings<br />
b Socket-outlet circuits in temporary <strong>installation</strong>s<br />
b Circuits supplying laundry rooms and swimming pools<br />
b Supply circuits to work-sites, caravans, pleasure boats, and travelling fairs<br />
See 2.2 and chapter P, al section 3<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
Fig. F59 : Fire-risk location<br />
2 y I rm y 4I n<br />
Great length <strong>of</strong> cable<br />
PE<br />
Fig. F60 : A circuit-breaker with low-set instantaneous<br />
magnetic trip<br />
Fire-risk<br />
location<br />
Phases<br />
Neutral<br />
PE<br />
7 Implementation <strong>of</strong> the IT system<br />
7.4 Protection in high fire-risk locations<br />
Protection by a RCD <strong>of</strong> sensitivity y 500 mA at the origin <strong>of</strong> the circuit supplying the<br />
fire-risk locations is mandatory in some countries (see Fig. F59).<br />
A preferred sensitivity <strong>of</strong> 300 mA may be adopted.<br />
7.5 When the fault current-loop impedance is<br />
particularly high<br />
When the earth-fault current is restricted due to an inevitably high fault-loop<br />
impedance, so that the overcurrent protection cannot be relied upon to trip the circuit<br />
within the prescribed time, the following possibilities should be considered:<br />
Suggestion 1 (see Fig. F60)<br />
b Install a circuit-breaker which has an instantaneous magnetic tripping element with<br />
an operation level which is lower than the usual setting, for example:<br />
2In y Irm y 4In<br />
This affords protection for persons on circuits which are abnormally long. It must<br />
be checked, however, that high transient currents such as the starting currents <strong>of</strong><br />
motors will not cause nuisance trip-outs.<br />
b Schneider Electric solutions<br />
v Type G Compact (2Im y Irm y 4Im)<br />
v Type B Multi 9 circuit-breaker<br />
Suggestion 2 (see Fig. F61)<br />
Install a RCD on the circuit. The device does not need to be highly-sensitive (HS)<br />
(several amps to a few tens <strong>of</strong> amps). Where socket-outlets are involved, the<br />
particular circuits must, in any case, be protected by HS (y 30 mA) RCDs; generally<br />
one RCD for a number <strong>of</strong> socket outlets on a common circuit.<br />
b Schneider Electric solutions<br />
v RCD Multi 9 NG125 : ΙΔn = 1 or 3 A<br />
v Vigicompact REH or REM: ΙΔn = 3 to 30 A<br />
Fig. F61 : RCD protection Fig. F62 : Improved equipotential bonding<br />
Suggestion 3<br />
Increase the size <strong>of</strong> the PE conductors and/or the phase conductors, to reduce the<br />
loop impedance.<br />
Suggestion 4 (see Fig. F62)<br />
Add supplementary equipotential conductors. This will have a similar effect to that<br />
<strong>of</strong> suggestion 3, i.e. a reduction in the earth-fault-loop resistance, while at the same<br />
time improving the existing touch-voltage protection measures. The effectiveness<br />
<strong>of</strong> this improvement may be checked by a resistance test between each exposed<br />
conductive part and the local main protective conductor.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
F35<br />
© Schneider Electric - all rights reserved
F36<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
Industrial circuit-breakers with an integrated<br />
RCD are covered in IEC 60947-2 and its<br />
appendix B<br />
Household or domestic circuit-breakers with<br />
an integrated RCD are covered in IEC 60898,<br />
IEC 61008 and IEC 61009<br />
8 Residual current devices (RCDs)<br />
8.1 Types <strong>of</strong> RCDs<br />
Residual current devices (RCD) are commonly incorporated in or associated with the<br />
following components:<br />
b Industrial-type moulded-case circuit-breakers (MCCB) conforming to IEC 60947-2<br />
and its appendix B and M<br />
b Industrial type miniature circuit-breakers (MCB) conforming to IEC 60947-2 and its<br />
appendix B and M<br />
b Household and similar miniature circuit-breakers (MCB) complying with IEC 60898,<br />
IEC 61008, IEC 61009<br />
b Residual load switch conforming to particular national standards<br />
b Relays with separate toroidal (ring-type) current transformers, conforming to<br />
IEC 60947-2 Appendix M<br />
RCDs are mandatorily used at the origin <strong>of</strong> TT-earthed <strong>installation</strong>s, where their<br />
ability to discriminate with other RCDs allows selective tripping, thereby ensuring the<br />
level <strong>of</strong> service continuity required.<br />
Industrial type circuit-breakers with integrated or adaptable<br />
RCD module (see Fig. F63)<br />
Industrial type circuit-breaker<br />
Vigi Compact<br />
Fig. F63 : Industrial-type CB with RCD module<br />
Fig. F64 : Domestic residual current circuit-breakers<br />
(RCCBs) for earth leakage protection<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Multi 9 DIN-rail industrial<br />
Circuit-breaker with adaptable Vigi RCD module<br />
Adaptable residual current circuit-breakers, including DIN-rail mounted units (e.g.<br />
Compact or Multi 9), are available, to which may be associated an auxiliary RCD<br />
module (e.g. Vigi).<br />
The ensemble provides a comprehensive range <strong>of</strong> protective functions (isolation,<br />
protection against short-circuit, overload, and earth-fault.<br />
Household and similar miniature circuit-breakers with RCD<br />
(see Fig. F64)<br />
The incoming-supply circuitbreaker<br />
can also have timedelayed<br />
characteristics and<br />
integrate a RCD (type S).<br />
“Monobloc” Déclic Vigi residual current circuit-breakers<br />
intended for protection <strong>of</strong> terminal socket-outlet circuits<br />
in domestic and tertiary sector applications.
F - Protection against electric shock<br />
Residual current load break switches are<br />
covered by particular national standards.<br />
RCDs with separate toroidal current<br />
transformers are standardized in IEC 60947-2<br />
appendix M<br />
I1 I2<br />
Fig. F66 : The principle <strong>of</strong> RCD operation<br />
I3<br />
8 Residual current devices (RCDs)<br />
Residual current circuit-breakers and RCDs with separate<br />
toroidal current transformer (see Fig. F65)<br />
RCDs with separate toroidal CTs can be used in association with circuit-breakers or<br />
contactors.<br />
Fig. F65 : RCDs with separate toroidal current transformers<br />
8.2 Description<br />
Principle<br />
The essential features are shown schematically in Figure F66 below.<br />
A magnetic core encompasses all the current-carrying conductors <strong>of</strong> an electric<br />
circuit and the magnetic flux generated in the core will depend at every instant on<br />
the arithmetical sum <strong>of</strong> the currents; the currents passing in one direction being<br />
considered as positive (Ι1), while those passing in the opposite direction will be<br />
negative (Ι2).<br />
In a normally healthy circuit Ι1 + Ι2 = 0 and there will be no flux in the magnetic core,<br />
and zero e.m.f. in its coil.<br />
An earth-fault current Ιd will pass through the core to the fault, but will return to the<br />
source via the earth, or via protective conductors in a TN-earthed system.<br />
The current balance in the conductors passing through the magnetic core therefore<br />
no longer exists, and the difference gives rise to a magnetic flux in the core.<br />
The difference current is known as the “residual” current and the principle is referred<br />
to as the “residual current” principle.<br />
The resultant alternating flux in the core induces an e.m.f. in its coil, so that a current<br />
I3 flows in the tripping-device operating coil. If the residual current exceeds the value<br />
required to operate the tripping device either directly or via an electronic relay, then<br />
the associated circuit-breaker will trip.<br />
8.3 Sensitivity <strong>of</strong> RDCs to disturbances<br />
In certain cases, aspects <strong>of</strong> the environment can disturb the correct operation <strong>of</strong><br />
RCDs:<br />
b “nuisance” tripping: Break in power supply without the situation being really<br />
hazardous. This type <strong>of</strong> tripping is <strong>of</strong>ten repetitive, causing major inconvenience and<br />
detrimental to the quality <strong>of</strong> the user's <strong>electrical</strong> power supply.<br />
b non-tripping, in the event <strong>of</strong> a hazard. Less perceptible than nuisance tripping,<br />
these malfunctions must still be examined carefully since they undermine user safety.<br />
This is why international standards define 3 categories <strong>of</strong> RCDs according to their<br />
immunity to this type <strong>of</strong> disturbance (see below).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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F38<br />
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F - Protection against electric shock<br />
100%<br />
90%<br />
10%<br />
60%<br />
Umax<br />
0.5U<br />
U<br />
1.2 s 50 s<br />
Fig. F68 : Standardized 1.2/50 µs voltage transient wave<br />
1<br />
0.9<br />
0.5<br />
0.1<br />
I<br />
I<br />
ca.0.5 s<br />
10 s (f = 100 kHz)<br />
Fig. F67 : Standardized 0.5 µs/100 kHz current transient wave<br />
Fig. F69 : Standardized current-impulse wave 8/20 µs<br />
t<br />
t<br />
t<br />
8 Residual current devices (RCDs)<br />
Main disturbance types<br />
Permanent earth leakage currents<br />
Every LV <strong>installation</strong> has a permanent leakage current to earth, which is either due<br />
to:<br />
b Unbalance <strong>of</strong> the intrinsic capacitance between live conductors and earth for threephase<br />
circuits or<br />
b Capacitance between live conductors and earth for single-phase circuits<br />
The larger the <strong>installation</strong> the greater its capacitance with consequently increased<br />
leakage current.<br />
The capacitive current to earth is sometimes increased significantly by filtering<br />
capacitors associated with electronic equipment (automation, IT and computerbased<br />
systems, etc.).<br />
In the absence <strong>of</strong> more precise data, permanent leakage current in a given<br />
<strong>installation</strong> can be estimated from the following values, measured at 230 V 50 Hz:<br />
Single-phase or three-phase line: 1.5 mA /100m<br />
b Heating floor: 1mA / kW<br />
b Fax terminal, printer: 1 mA<br />
b Microcomputer, workstation: 2 mA<br />
b Copy machine: 1.5 mA<br />
Since RCDs complying with IEC and many national standards may operate under,<br />
the limitation <strong>of</strong> permanent leakage current to 0.25 IΔn, by sub-division <strong>of</strong> circuits<br />
will, in practice, eliminate any unwanted tripping.<br />
For very particular cases, such as the extension, or partial renovation <strong>of</strong> extended<br />
IT-earthed <strong>installation</strong>s, the manufacturers must be consulted.<br />
High frequency components (harmonics, transients, etc.), are found in computer<br />
equipment power supplies, converters, motors with speed regulators, fluorescent<br />
lighting systems and in the vicinity <strong>of</strong> high power switching devices and reactive<br />
energy compensation banks.<br />
Part <strong>of</strong> these high frequency currents may flow to earth through parasite<br />
capacitances (line, units, etc.). Although not hazardous for the user, these currents<br />
can still cause the tripping <strong>of</strong> differential devices.<br />
Energization<br />
The initial energization <strong>of</strong> the capacitances mentioned above gives rise to high<br />
frequency transient currents <strong>of</strong> very short duration, similar to that shown in<br />
Figure F67.<br />
The sudden occurrence <strong>of</strong> a first-fault on an IT-earthed system also causes transient<br />
earth-leakage currents at high frequency, due to the sudden rise <strong>of</strong> the two healthy<br />
phases to phase/phase voltage above earth.<br />
Common mode overvoltages<br />
Electrical networks are subjected to overvoltages due to lightning strikes or to abrupt<br />
changes <strong>of</strong> system operating conditions (faults, fuse operation, switching, etc.).<br />
These sudden changes <strong>of</strong>ten cause large transient voltages and currents in inductive<br />
and capacitive circuits. Records have established that, on LV systems, overvoltages<br />
remain generally below 6 kV, and that they can be adequately represented by the<br />
conventional 1.2/50 μs impulse wave (see Fig. F68).<br />
These overvoltages give rise to transient currents represented by a current impulse<br />
wave <strong>of</strong> the conventional 8/20 μs form, having a peak value <strong>of</strong> several tens <strong>of</strong><br />
amperes (see Fig. F69).<br />
The transient currents flow to earth via the capacitances <strong>of</strong> the <strong>installation</strong>.<br />
Non-sinusoidal fault currents<br />
An RCD must be selected taking into account the type <strong>of</strong> supplied load. This applies<br />
in particular to semiconductor-based devices for which fault currents are not always<br />
sinusoidal.<br />
Type AC, A, B<br />
Standard IEC 60755 (<strong>General</strong> requirements for residual current operated protective<br />
devices) defines three types <strong>of</strong> RCD depending on the characteristics <strong>of</strong> the fault<br />
current:<br />
b Type AC<br />
RCD for which tripping is ensured for residual sinusoidal alternating currents.<br />
b Type A<br />
RCD for which tripping is ensured:<br />
v for residual sinusoidal alternating currents,<br />
v for residual pulsating direct currents,<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
F - Protection against electric shock<br />
8 Residual current devices (RCDs)<br />
b Type B<br />
RCD for which tripping is ensured:<br />
v as for type A,<br />
v for pure direct residual currents which may result from three-phase rectifying<br />
circuits.<br />
Cold: in the cases <strong>of</strong> temperatures under - 5 °C, very high sensitivity<br />
electromechanical relays in the RCD may be “welded” by the condensation – freezing<br />
action.<br />
Type “Si” devices can operate under temperatures down to - 25 °C.<br />
Atmospheres with high concentrations <strong>of</strong> chemicals or dust: the special alloys<br />
used to make the RCDs can be notably damaged by corrosion. Dust can also block<br />
the movement <strong>of</strong> mechanical parts.<br />
See the measures to be taken according to the levels <strong>of</strong> severity defined by<br />
standards in Fig. F70.<br />
Regulations define the choice <strong>of</strong> earth leakage protection and its implementation.<br />
The main reference texts are as follows:<br />
b Standard IEC 60364-3:<br />
v This gives a classification (AFx) for external influences in the presence <strong>of</strong> corrosive<br />
or polluting substances.<br />
v It defines the choice <strong>of</strong> materials to be used according to extreme influences.<br />
Disturbed<br />
network<br />
Influence <strong>of</strong><br />
the <strong>electrical</strong><br />
network<br />
Clean network<br />
Superimmunized<br />
residual current<br />
protections<br />
Type A if: k<br />
Standard<br />
immunized<br />
residual current<br />
protections<br />
Type AC<br />
Fig. F70 : External influence classification according to IEC 60364-3 standard<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
SiE k<br />
residual current<br />
protections<br />
SiE k<br />
residual current<br />
protections<br />
+<br />
Appropriate<br />
additional<br />
protection<br />
(sealed cabinet<br />
or unit)<br />
AF1 AF2 AF3 AF4<br />
b External<br />
influences:<br />
negligible,<br />
b Equipment<br />
characteristics:<br />
normal.<br />
b External<br />
influences:<br />
presence<br />
<strong>of</strong> corrosive<br />
or polluting<br />
atmospheric<br />
agents,<br />
b Equipment<br />
characteristics:<br />
e.g. conformity<br />
with salt mist<br />
or atmospheric<br />
pollution tests.<br />
b External<br />
influences:<br />
intermittent<br />
or accidental<br />
action <strong>of</strong><br />
certain<br />
common<br />
chemicals,<br />
b Equipment<br />
characteristics:<br />
corrosion<br />
protection.<br />
Examples <strong>of</strong> exposed sites External influences<br />
SiE k<br />
residual current<br />
protections<br />
+<br />
Appropriate<br />
additional<br />
protection<br />
(sealed cabinet<br />
or unit +<br />
overpressure)<br />
b External<br />
influences:<br />
permanent<br />
action <strong>of</strong><br />
corrosive<br />
or polluting<br />
chemicals<br />
b Equipment<br />
characteristics:<br />
specifically<br />
studied<br />
according to<br />
the type <strong>of</strong><br />
products.<br />
Iron and steel works. Presence <strong>of</strong> sulfur, sulfur vapor, hydrogen<br />
sulfide.<br />
Marinas, trading ports, boats, sea edges, naval<br />
shipyards.<br />
Salt atmospheres, humid outside, low<br />
temperatures.<br />
Swimming pools, hospitals, food & beverage. Chlorinated compounds.<br />
Petrochemicals. Hydrogen, combustion gases, nitrogen<br />
oxides.<br />
Breeding facilities, tips. Hydrogen sulfide.<br />
F39<br />
© Schneider Electric - all rights reserved
F40<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
8 Residual current devices (RCDs)<br />
Immunity level for Merlin Gerin residual current devices<br />
The Merlin Gerin range comprises various types <strong>of</strong> RCDs allowing earth leakage<br />
protection to be adapted to each application. The table below indicates the choices to<br />
be made according to the type <strong>of</strong> probable disturbances at the point <strong>of</strong> <strong>installation</strong>.<br />
Device type Nuisance<br />
trippings<br />
High<br />
frequency<br />
leakage<br />
current<br />
AC b<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Non-trippings<br />
Fault current Low<br />
Rectified<br />
alternating<br />
Pure direct<br />
temperatures<br />
(down to<br />
- 25 °C)<br />
A b b b<br />
SI b b b b b<br />
Corrosion<br />
Dust<br />
SiE b b b b b b<br />
B b b b b b b<br />
Fig. F71 : Immunity level <strong>of</strong> Merlin Gerin RCDs<br />
Immunity to nuisance tripping<br />
Type Si/SiE RCDs have been <strong>design</strong>ed to avoid nuisance tripping or non-tripping in<br />
case <strong>of</strong> polluted network , lightning effect, high frequency currents, RF waves, etc.<br />
Figure F72 below indicates the levels <strong>of</strong> tests undergone by this type <strong>of</strong> RCDs.<br />
Disturbance type Rated test wave Immunity<br />
Continuous disturbances<br />
Multi9 :<br />
ID-RCCB, DPN Vigi, Vigi C60, Vigi<br />
C120, Vigi NG125<br />
SI / SiE type<br />
Harmonics 1 kHz Earth leakage current = 8 x I∆n<br />
Transient disturbances<br />
Lightning induced overvoltage 1.2 / 50 μs pulse<br />
(IEC/EN 61000-4-5)<br />
Lightning induced current 8 / 20 μs pulse<br />
(IEC/EN 61008)<br />
Switching transient, indirect<br />
lightning currents<br />
Downstream surge arrester<br />
operation, capacitance loading<br />
Electromagnetic compatibility<br />
Inductive load switchings<br />
fluorescent lights, motors, etc.)<br />
Fluorescent lights, thyristor<br />
controlled circuits, etc.<br />
RF waves (TV&<br />
radio, broadcact,<br />
telecommunications,etc.)<br />
0.5 μs / 100 kHz<br />
“ ring wave ”<br />
(IEC/EN 61008)<br />
4.5 kV between conductors 5.5 kV /<br />
earth<br />
5 kA peak<br />
400 A peak<br />
10 ms pulse 500 A<br />
Repeated bursts<br />
(IEC 61000-4-4)<br />
RF conducted<br />
waves<br />
(IEC 61000-4-6)<br />
RF radiated waves<br />
80 MHz to 1 GHz<br />
(IEC 61000-4-3)<br />
4 kV / 400 kHz<br />
66 mA (15 kHz to 150 kHz)<br />
30 V (150 kHz to 230 MHz)<br />
30 V / m<br />
Fig. F72 : Immunity to nuisance tripping tests undergone by Merlin Gerin RCDs
F - Protection against electric shock<br />
8 Residual current devices (RCDs)<br />
Recommendations concerning the <strong>installation</strong> <strong>of</strong> RCDs with<br />
separate toroidal current transformers<br />
The detector <strong>of</strong> residual current is a closed magnetic circuit (usually circular) <strong>of</strong><br />
very high magnetic permeability, on which is wound a coil <strong>of</strong> wire, the ensemble<br />
constituting a toroidal (or ring-type) current transformer.<br />
Because <strong>of</strong> its high permeability, any small deviation from perfect symmetry <strong>of</strong><br />
the conductors encompassed by the core, and the proximity <strong>of</strong> ferrous material<br />
(steel enclosure, chassis members, etc.) can affect the balance <strong>of</strong> magnetic forces<br />
sufficiently, at times <strong>of</strong> large load currents (motor-starting current, transformer<br />
energizing current surge, etc.) to cause unwanted tripping <strong>of</strong> the RCD.<br />
Unless particular measures are taken, the ratio <strong>of</strong> operating current Ι∆n to maximum<br />
phase current Ιph (max.) is generally less than 1/1,000.<br />
This limit can be increased substantially (i.e. the response can be desensitized) by<br />
adopting the measures shown in Figure F73 , and summarized in Figure F74.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
L = twice the diameter <strong>of</strong><br />
the magnetic ring core<br />
Fig. F73 : Three measures to reduce the ratio Ι∆n/Ιph (max.)<br />
Measures Diameter Sensitivity<br />
(mm) diminution factor<br />
Careful centralizing <strong>of</strong> cables through the ring core 3<br />
Oversizing <strong>of</strong> the ring core ø 50 → ø 100 2<br />
ø 80 → ø 200 2<br />
ø 120 → ø 300 6<br />
Use <strong>of</strong> a steel or s<strong>of</strong>t-iron shielding sleeve ø 50 4<br />
b Of wall thickness 0.5 mm ø 80 3<br />
b Of length 2 x inside diameter <strong>of</strong> ring core ø 120 3<br />
b Completely surrounding the conductors and ø 200 2<br />
overlapping the circular core equally at both ends<br />
These measures can be combined. By carefully centralizing the cables in a ring core<br />
<strong>of</strong> 200 mm diameter, where a 50 mm core would be large enough, and using a sleeve,<br />
the ratio 1/1,000 could become 1/30,000.<br />
Fig. F74 : Means <strong>of</strong> reducing the ratio IΔn/Iph (max.)<br />
L<br />
F41<br />
© Schneider Electric - all rights reserved
F42<br />
© Schneider Electric - all rights reserved<br />
F - Protection against electric shock<br />
a b<br />
In<br />
In1<br />
Fig. F75 : Residual current circuit-breakers (RCCBs)<br />
In<br />
In1 In2 In3 In4<br />
Choice <strong>of</strong> characteristics <strong>of</strong> a residual-current circuit-breaker<br />
(RCCB - IEC 61008)<br />
Rated current<br />
The rated current <strong>of</strong> a RCCB is chosen according to the maximum sustained load<br />
current it will carry.<br />
b If the RCCB is connected in series with, and downstream <strong>of</strong> a circuit-breaker, the<br />
rated current <strong>of</strong> both items will be the same, i.e. Ιn u Ιn1 (1) (see Fig. F75a)<br />
b If the RCCB is located upstream <strong>of</strong> a group <strong>of</strong> circuits, protected by circuitbreakers,<br />
as shown in Figure F75b, then the RCCB rated current will be given by:<br />
Ιn u ku x ks (Ιn1 + Ιn2 + Ιn3 + Ιn4)<br />
Circuit-breaker and RCCB association – maxi Ιsc (r.m.s) value in kA<br />
Electrodynamic withstand requirements<br />
Protection against short-circuits must be provided by an upstream SCPD (Short-<br />
Circuit Protective Device) but it is considered that where the RCCB is located in the<br />
same distribution box (complying with the appropriate standards) as the downstream<br />
circuit-breakers (or fuses), the short-circuit protection afforded by these (outgoingcircuit)<br />
SCPDs is an adequate alternative. Coordination between the RCCB and the<br />
SCPDs is necessary, and manufacturers generally provide tables associating RCCBs<br />
and circuit-breakers or fuses (see Fig. F76).<br />
Upstream circuit-breaker DT40 DT40N C60N C60H C60L C120N C120H NG125N NG125H<br />
Downstream 2P I 20A 6.5 6.5 6.5 6.5 6.5 3 4.5 4.5 4.5<br />
RCCB 230V IN-A 40A 6 10 20 30 30 10 10 15 15<br />
IN-A 63A 6 10 20 30 30 10 10 15 15<br />
I 100A 15 15 15 15<br />
4P I 20A 4.5 4.5 4.5 4.5 4.5 2 3 3 3<br />
400V IN-A 40A 6 10 10 15 15 7 7 15 15<br />
IN-A 63A 6 10 10 15 15 7 7 15 15<br />
NG 125NA 10 16 25 50<br />
Fuses and RCCB association – maxi Ιsc (r.m.s) value in kA<br />
gG upstream fuse 20A 63A 100A 125A<br />
Downstream 2P I 20A 8<br />
RCCB 230V IN-A 40A 30 20<br />
IN-A 63A 30 20<br />
I 100A 6<br />
4P I 20A 8<br />
400V IN-A 40A 30 20<br />
IN-A 63A 30 20<br />
NG 125NA 50<br />
Fig. F76 : Typical manufacturers coordination table for RCCBs, circuit-breakers, and fuses (Merlin Gerin products)<br />
8 Residual current devices (RCDs)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
<strong>Chapter</strong> G<br />
Sizing and protection <strong>of</strong> conductors<br />
Contents<br />
<strong>General</strong> G2<br />
1.1 Methodology and definition G2<br />
1.2 Overcurrent protection principles G4<br />
1.3 Practical values for a protective scheme G4<br />
1.4 Location <strong>of</strong> protective devices G6<br />
1.5 Conductors in parallel G6<br />
Practical method for determining the smallest allowable G7<br />
cross-sectional area <strong>of</strong> circuit conductors<br />
2.1 <strong>General</strong> G7<br />
2.2 <strong>General</strong> method for cables G7<br />
2.3 Recommended simplified approach for cables G16<br />
2.4 Busbar trunking systems G18<br />
Determination <strong>of</strong> voltage drop G20<br />
3.1 Maximum voltage drop limit G20<br />
3.2 Calculation <strong>of</strong> voltage drop in steady load conditions G21<br />
Short-circuit current G24<br />
4.1 Short-circuit current at the secondary terminals <strong>of</strong> G24<br />
a MV/LV distribution transformer<br />
4.2 3-phase short-circuit current (Isc) at any point within G25<br />
a LV <strong>installation</strong><br />
4.3 Isc at the receiving end <strong>of</strong> a feeder in terms <strong>of</strong> the Isc G28<br />
at its sending end<br />
4.4 Short-circuit current supplied by an alternator or an inverter G29<br />
Particular cases <strong>of</strong> short-circuit current G30<br />
5.1 Calculation <strong>of</strong> minimum levels <strong>of</strong> short-circuit current G30<br />
5.2 Verification <strong>of</strong> the withstand capabilities <strong>of</strong> cables under G35<br />
short-circuit conditions<br />
Protective earthing conductor G37<br />
6.1 Connection and choice G37<br />
6.2 Conductor sizing G38<br />
6.3 Protective conductor between MV/LV transformer and G40<br />
the main general distribution board (MGDB)<br />
6.4 Equipotential conductor G41<br />
The neutral conductor G42<br />
7.1 Sizing the neutral conductor G42<br />
7.2 Protection <strong>of</strong> the neutral conductor G42<br />
7.3 Breaking <strong>of</strong> the neutral conductor G44<br />
7.4 Isolation <strong>of</strong> the neutral conductor G44<br />
Worked example <strong>of</strong> cable calculation G46<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
G2<br />
G - Sizing and protection <strong>of</strong> conductors<br />
Component parts <strong>of</strong> an electric circuit and its<br />
protection are determined such that all normal<br />
and abnormal operating conditions are satisfied<br />
(1) The term “cabling” in this chapter, covers all insulated<br />
conductors, including multi-core and single-core cables and<br />
insulated wires drawn into conduits, etc.<br />
<strong>General</strong><br />
. Methodology and definition<br />
Methodology (see Fig. G )<br />
Following a preliminary analysis <strong>of</strong> the power requirements <strong>of</strong> the <strong>installation</strong>, as<br />
described in <strong>Chapter</strong> B Clause 4, a study <strong>of</strong> cabling (1) and its <strong>electrical</strong> protection is<br />
undertaken, starting at the origin <strong>of</strong> the <strong>installation</strong>, through the intermediate stages<br />
to the final circuits.<br />
The cabling and its protection at each level must satisfy several conditions at the<br />
same time, in order to ensure a safe and reliable <strong>installation</strong>, e.g. it must:<br />
b Carry the permanent full load current, and normal short-time overcurrents<br />
b Not cause voltage drops likely to result in an inferior performance <strong>of</strong> certain loads,<br />
for example: an excessively long acceleration period when starting a motor, etc.<br />
Moreover, the protective devices (circuit-breakers or fuses) must:<br />
b Protect the cabling and busbars for all levels <strong>of</strong> overcurrent, up to and including<br />
short-circuit currents<br />
b Ensure protection <strong>of</strong> persons against indirect contact hazards, particularly in<br />
TN- and IT- earthed systems, where the length <strong>of</strong> circuits may limit the magnitude<br />
<strong>of</strong> short-circuit currents, thereby delaying automatic disconnection (it may be<br />
remembered that TT- earthed <strong>installation</strong>s are necessarily protected at the origin by<br />
a RCD, generally rated at 300 mA).<br />
The cross-sectional areas <strong>of</strong> conductors are determined by the general method<br />
described in Sub-clause 2 <strong>of</strong> this <strong>Chapter</strong>. Apart from this method some national<br />
standards may prescribe a minimum cross-sectional area to be observed for reasons<br />
<strong>of</strong> mechanical endurance. Particular loads (as noted in <strong>Chapter</strong> N) require that the<br />
cable supplying them be oversized, and that the protection <strong>of</strong> the circuit be likewise<br />
modified.<br />
Fig. G1 : Flow-chart for the selection <strong>of</strong> cable size and protective device rating for a given circuit<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Power demand:<br />
- kVA to be supplied<br />
- Maximum load current I B<br />
Conductor sizing:<br />
- Selection <strong>of</strong> conductor type and insulation<br />
- Selection <strong>of</strong> method <strong>of</strong> <strong>installation</strong><br />
- Taking account <strong>of</strong> correction factors for<br />
different environment conditions<br />
- Determination <strong>of</strong> cross-sectional areas using<br />
tables giving the current carrying capability<br />
Verification <strong>of</strong> the maximum voltage drop:<br />
- Steady state conditions<br />
- Motor starting conditions<br />
Calculation <strong>of</strong> short-circuit currents:<br />
- Upstream short-circuit power<br />
- Maximum values<br />
- Minimum values at conductor end<br />
Selection <strong>of</strong> protective devices:<br />
- Rated current<br />
- Breaking capability<br />
- Implementation <strong>of</strong> cascading<br />
- Check <strong>of</strong> discrimination
G - Sizing and protection <strong>of</strong> conductors<br />
<strong>General</strong><br />
Definitions<br />
Maximum load current: IB<br />
b At the final circuits level, this current corresponds to the rated kVA <strong>of</strong> the load.<br />
In the case <strong>of</strong> motor-starting, or other loads which take a high in-rush current,<br />
particularly where frequent starting is concerned (e.g. lift motors, resistance-type<br />
spot welding, and so on) the cumulative thermal effects <strong>of</strong> the overcurrents must be<br />
taken into account. Both cables and thermal type relays are affected.<br />
b At all upstream circuit levels this current corresponds to the kVA to be supplied,<br />
which takes account <strong>of</strong> the factors <strong>of</strong> simultaneity (diversity) and utilization, ks and ku<br />
respectively, as shown in Figure G2.<br />
80 A 60 A 100 A<br />
Fig. G2 : Calculation <strong>of</strong> maximum load current IB<br />
Maximum permissible current: Iz<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Main distribution<br />
board<br />
Combined factors <strong>of</strong> simultaneity<br />
(or diversity) and utilization:<br />
ks x ku = 0.69<br />
IB = (80+60+100+50) x 0.69 = 200 A<br />
M<br />
Sub-distribution<br />
board<br />
50 A<br />
Normal load<br />
motor current<br />
50 A<br />
This is the maximum value <strong>of</strong> current that the cabling for the circuit can carry<br />
indefinitely, without reducing its normal life expectancy.<br />
The current depends, for a given cross sectional area <strong>of</strong> conductors, on several<br />
parameters:<br />
b Constitution <strong>of</strong> the cable and cable-way (Cu or Alu conductors; PVC or EPR etc.<br />
insulation; number <strong>of</strong> active conductors)<br />
b Ambient temperature<br />
b Method <strong>of</strong> <strong>installation</strong><br />
b Influence <strong>of</strong> neighbouring circuits<br />
Overcurrents<br />
An overcurrent occurs each time the value <strong>of</strong> current exceeds the maximum load<br />
current IB for the load concerned.<br />
This current must be cut <strong>of</strong>f with a rapidity that depends upon its magnitude, if<br />
permanent damage to the cabling (and appliance if the overcurrent is due to a<br />
defective load component) is to be avoided.<br />
Overcurrents <strong>of</strong> relatively short duration can however, occur in normal operation; two<br />
types <strong>of</strong> overcurrent are distinguished:<br />
b Overloads<br />
These overcurrents can occur in healthy electric circuits, for example, due to a<br />
number <strong>of</strong> small short-duration loads which occasionally occur co-incidentally: motor<br />
starting loads, and so on. If either <strong>of</strong> these conditions persists however beyond a<br />
given period (depending on protective-relay settings or fuse ratings) the circuit will be<br />
automatically cut <strong>of</strong>f.<br />
b Short-circuit currents<br />
These currents result from the failure <strong>of</strong> insulation between live conductors or/and<br />
between live conductors and earth (on systems having low-impedance-earthed<br />
neutrals) in any combination, viz:<br />
v 3 phases short-circuited (and to neutral and/or earth, or not)<br />
v 2 phases short-circuited (and to neutral and/or earth, or not)<br />
v 1 phase short-circuited to neutral (and/or to earth)<br />
G3<br />
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© Schneider Electric - all rights reserved<br />
G4<br />
G - Sizing and protection <strong>of</strong> conductors<br />
t<br />
Fig. G3 : Circuit protection by circuit-breaker<br />
t<br />
Maximum<br />
load<br />
current<br />
Temporary<br />
overload<br />
Temporary<br />
overload<br />
I 2 t cable<br />
characteristic<br />
IB Ir Iz<br />
ISCB<br />
IB Ir cIz Iz<br />
Fig. G4 : Circuit protection by fuses<br />
Circuit-breaker<br />
tripping curve<br />
ICU<br />
I 2 t cable<br />
characteristic<br />
Fuse curve<br />
(1) Both <strong>design</strong>ations are commonly used in different<br />
standards.<br />
I<br />
I<br />
<strong>General</strong><br />
.2 Overcurrent protection principles<br />
A protective device is provided at the origin <strong>of</strong> the circuit concerned (see Fig. G3 and<br />
Fig. G4).<br />
b Acting to cut-<strong>of</strong>f the current in a time shorter than that given by the I 2 t<br />
characteristic <strong>of</strong> the circuit cabling<br />
b But allowing the maximum load current IB to flow indefinitely<br />
The characteristics <strong>of</strong> insulated conductors when carrying short-circuit currents<br />
can, for periods up to 5 seconds following short-circuit initiation, be determined<br />
approximately by the formula:<br />
I 2 t = k 2 S 2 which shows that the allowable heat generated is proportional to the<br />
squared cross-sectional-area <strong>of</strong> the condutor.<br />
where<br />
t: Duration <strong>of</strong> short-circuit current (seconds)<br />
S: Cross sectional area <strong>of</strong> insulated conductor (mm 2 )<br />
I: Short-circuit current (A r.m.s.)<br />
k: Insulated conductor constant (values <strong>of</strong> k 2 are given in Figure G52 )<br />
For a given insulated conductor, the maximum permissible current varies according<br />
to the environment. For instance, for a high ambient temperature (θa1 > θa2), Iz1 is<br />
less than Iz2 (see Fig. G5). θ means “temperature”.<br />
Note:<br />
v ISC: 3-phase short-circuit current<br />
v ISCB: rated 3-ph. short-circuit breaking current <strong>of</strong> the circuit-breaker<br />
v Ir (or Irth) (1) : regulated “nominal” current level; e.g. a 50 A nominal circuit-breaker<br />
can be regulated to have a protective range, i.e. a conventional overcurrent tripping<br />
level (see Fig. G6 opposite page) similar to that <strong>of</strong> a 30 A circuit-breaker.<br />
.3 Practical values for a protective scheme<br />
The following methods are based on <strong>rules</strong> laid down in the IEC standards, and are<br />
representative <strong>of</strong> the practices in many countries.<br />
<strong>General</strong> <strong>rules</strong><br />
A protective device (circuit-breaker or fuse) functions correctly if:<br />
b Its nominal current or its setting current In is greater than the maximum load<br />
current IB but less than the maximum permissible current Iz for the circuit, i.e.<br />
IB y In y Iz corresponding to zone “a” in Figure G6<br />
b Its tripping current I2 “conventional” setting is less than 1.45 Iz which corresponds<br />
to zone “b” in Figure G6<br />
The “conventional” setting tripping time may be 1 hour or 2 hours according to local<br />
standards and the actual value selected for I2. For fuses, I2 is the current (denoted<br />
If) which will operate the fuse in the conventional time.<br />
5 s<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
t<br />
1 2<br />
Iz1 < Iz2<br />
θa1 > θa2<br />
I 2 t = k 2 S 2<br />
Fig. G5 : I 2 t characteristic <strong>of</strong> an insulated conductor at two different ambient temperatures<br />
I
G - Sizing and protection <strong>of</strong> conductors<br />
0<br />
<strong>General</strong><br />
Loads Circuit cabling<br />
Maximum load current IB<br />
IB<br />
Nominal current In or<br />
its regulated current Ir<br />
I n<br />
zone a<br />
Fig. G6 : Current levels for determining circuir breaker or fuse characteristics<br />
Criteria for circuit-breakers:<br />
IB y In y Iz and ISCB u ISC.<br />
Criteria for fuses:<br />
IB y In y Iz/k3 and ISCF u ISC.<br />
Iz<br />
Maximum load current Iz<br />
zone b<br />
Protective device<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
I2<br />
Conventional overcurrent<br />
trip current I2<br />
1.45 Iz<br />
1.45 Iz Isc<br />
ISCB<br />
zone c<br />
3-ph short-circuit<br />
fault-current breaking rating<br />
IB y In y Iz zone a<br />
I2 y 1.45 Iz zone b<br />
ISCB u ISC zone c<br />
b Its 3-phase short-circuit fault-current breaking rating is greater than the 3-phase<br />
short-circuit current existing at its point <strong>of</strong> <strong>installation</strong>. This corresponds to zone “c” in<br />
Figure G6.<br />
Applications<br />
b Protection by circuit-breaker<br />
By virtue <strong>of</strong> its high level <strong>of</strong> precision the current I2 is always less than 1.45 In (or<br />
1.45 Ir) so that the condition I2 y 1.45 Iz (as noted in the “general <strong>rules</strong>” above) will<br />
always be respected.<br />
v Particular case<br />
If the circuit-breaker itself does not protect against overloads, it is necessary to<br />
ensure that, at a time <strong>of</strong> lowest value <strong>of</strong> short-circuit current, the overcurrent device<br />
protecting the circuit will operate correctly. This particular case is examined in Subclause<br />
5.1.<br />
b Protection by fuses<br />
The condition I2 y 1.45 Iz must be taken into account, where I2 is the fusing (melting<br />
level) current, equal to k2 x In (k2 ranges from 1.6 to 1.9) depending on the particular<br />
fuse concerned.<br />
A further factor k3 has been introduced ( k = k2<br />
3 ) such that I2 y 1.45 Iz<br />
1.45<br />
will be valid if In y Iz/k3.<br />
For fuses type gG:<br />
In < 16 A → k3 = 1.31<br />
In u 16 A → k3 = 1.10<br />
Moreover, the short-circuit current breaking capacity <strong>of</strong> the fuse ISCF must exceed<br />
the level <strong>of</strong> 3-phase short-circuit current at the point <strong>of</strong> <strong>installation</strong> <strong>of</strong> the fuse(s).<br />
b Association <strong>of</strong> different protective devices<br />
The use <strong>of</strong> protective devices which have fault-current ratings lower than the fault<br />
level existing at their point <strong>of</strong> <strong>installation</strong> are permitted by IEC and many national<br />
standards in the following conditions:<br />
v There exists upstream, another protective device which has the necessary shortcircuit<br />
rating, and<br />
v The amount <strong>of</strong> energy allowed to pass through the upstream device is less than<br />
that which can be withstood without damage by the downstream device and all<br />
associated cabling and appliances.<br />
G5<br />
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© Schneider Electric - all rights reserved<br />
G6<br />
G - Sizing and protection <strong>of</strong> conductors<br />
A protective device is, in general, required at the<br />
origin <strong>of</strong> each circuit<br />
a<br />
b<br />
c<br />
B<br />
Fig. G7 : Location <strong>of</strong> protective devices<br />
P<br />
P2 P3 P4<br />
50 mm 2 10 mm 2 25 mm 2<br />
< 3 m<br />
P2<br />
A<br />
B<br />
P 1<br />
P3<br />
Case (1) Case (2)<br />
P 1: C60 rated 15 A<br />
2.5 mm 2<br />
sc<br />
Short-circuit<br />
protective<br />
device<br />
B<br />
s<br />
Overload<br />
protective<br />
device<br />
Case (3)<br />
S 2:<br />
1.5 mm2<br />
<strong>General</strong><br />
In pratice this arrangement is generally exploited in:<br />
v The association <strong>of</strong> circuit-breakers/fuses<br />
v The technique known as “cascading” or “series rating” in which the strong<br />
current-limiting performance <strong>of</strong> certain circuit-breakers effectively reduces the<br />
severity <strong>of</strong> downstream short-circuits<br />
Possible combinations which have been tested in laboratories are indicated in certain<br />
manufacturers catalogues.<br />
.4 Location <strong>of</strong> protective devices<br />
<strong>General</strong> rule (see Fig. G7a)<br />
A protective device is necessary at the origin <strong>of</strong> each circuit where a reduction <strong>of</strong><br />
permissible maximum current level occurs.<br />
Possible alternative locations in certain circumstances<br />
(see Fig. G7b)<br />
The protective device may be placed part way along the circuit:<br />
b If AB is not in proximity to combustible material, and<br />
b If no socket-outlets or branch connections are taken from AB<br />
Three cases may be useful in practice:<br />
b Consider case (1) in the diagram<br />
v AB y 3 metres, and<br />
v AB has been installed to reduce to a practical minimum the risk <strong>of</strong> a short-circuit<br />
(wires in heavy steel conduit for example)<br />
b Consider case (2)<br />
v The upstream device P1 protects the length AB against short-circuits in<br />
accordance with Sub-clause 5.1<br />
b Consider case (3)<br />
v The overload device (S) is located adjacent to the load. This arrangement is<br />
convenient for motor circuits. The device (S) constitutes the control (start/stop) and<br />
overload protection <strong>of</strong> the motor while (SC) is: either a circuit-breaker (<strong>design</strong>ed for<br />
motor protection) or fuses type aM<br />
v The short-circuit protection (SC) located at the origin <strong>of</strong> the circuit conforms with<br />
the principles <strong>of</strong> Sub-clause 5.1<br />
Circuits with no protection (see Fig. G7c)<br />
Either<br />
b The protective device P1 is calibrated to protect the cable S2 against overloads<br />
and short-circuits<br />
Or<br />
b Where the breaking <strong>of</strong> a circuit constitutes a risk, e.g.<br />
v Excitation circuits <strong>of</strong> rotating machines<br />
v circuits <strong>of</strong> large lifting electromagnets<br />
v the secondary circuits <strong>of</strong> current transformers<br />
No circuit interruption can be tolerated, and the protection <strong>of</strong> the cabling is <strong>of</strong><br />
secondary importance.<br />
.5 Conductors in parallel<br />
Conductors <strong>of</strong> the same cross-sectional-area, the same length, and <strong>of</strong> the same<br />
material, can be connected in parallel.<br />
The maximum permissible current is the sum <strong>of</strong> the individual-core maximum<br />
currents, taking into account the mutual heating effects, method <strong>of</strong> <strong>installation</strong>, etc.<br />
Protection against overload and short-circuits is identical to that for a single-cable<br />
circuit.<br />
The following precautions should be taken to avoid the risk <strong>of</strong> short-circuits on the<br />
paralleled cables:<br />
b Additional protection against mechanical damage and against humidity, by the<br />
introduction <strong>of</strong> supplementary protection<br />
b The cable route should be chosen so as to avoid close proximity to combustible<br />
materials<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
2.1 <strong>General</strong><br />
The reference international standard for the study <strong>of</strong> cabling is IEC 60364-5-52:<br />
“Electrical <strong>installation</strong> <strong>of</strong> buildings - Part 5-52: Selection and erection <strong>of</strong> <strong>electrical</strong><br />
equipment - Wiring system”.<br />
A summary <strong>of</strong> this standard is presented here, with examples <strong>of</strong> the most commonly<br />
used methods <strong>of</strong> <strong>installation</strong>. The current-carrying capacities <strong>of</strong> conductors in all<br />
different situations are given in annex A <strong>of</strong> the standard. A simplified method for use<br />
<strong>of</strong> the tables <strong>of</strong> annex A is proposed in informative annex B <strong>of</strong> the standard.<br />
2.2 <strong>General</strong> method for cables<br />
Possible methods <strong>of</strong> <strong>installation</strong> for different types <strong>of</strong><br />
conductors or cables<br />
The different admissible methods <strong>of</strong> <strong>installation</strong> are listed in Figure G8, in<br />
conjonction with the different types <strong>of</strong> conductors and cables.<br />
Conductors and cables Method <strong>of</strong> <strong>installation</strong><br />
Without Clipped Conduit Cable trunking Cable Cable ladder On Support<br />
fixings direct (including ducting Cable tray insulators wire<br />
skirting trunking, Cable brackets<br />
flush floor trunking)<br />
Bare conductors – – – – – – + –<br />
Insulated conductors – – + + + – + –<br />
Sheathed Multi-core + + + + + + 0 +<br />
cables<br />
(including<br />
armoured Single-core 0 + + + + + 0 +<br />
and<br />
mineral<br />
insulated)<br />
+ Permitted.<br />
– Not permitted.<br />
0 Not applicable, or not normally used in practice.<br />
Fig. G8 : Selection <strong>of</strong> wiring systems (table 52-1 <strong>of</strong> IEC 60364-5-52)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
G8<br />
G - Sizing and protection <strong>of</strong> conductors<br />
Fig. G9 : Erection <strong>of</strong> wiring systems (table 52-2 <strong>of</strong> IEC 60364-5-52)<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
Possible methods <strong>of</strong> <strong>installation</strong> for different situations:<br />
Different methods <strong>of</strong> <strong>installation</strong> can be implemented in different situations. The<br />
possible combinations are presented in Figure G9.<br />
The number given in this table refer to the different wiring systems considered.<br />
(see also Fig. G10)<br />
Situations Method <strong>of</strong> <strong>installation</strong><br />
Without With Conduit Cable trunking Cable Cable ladder On Support<br />
fixings fixings (including ducting cable tray, insulators wire<br />
skirting trunking,<br />
flush floor trunking)<br />
cable brackets<br />
Building voids 40, 46, 0 15, 16, – 43 30, 31, 32, – –<br />
15, 16 41, 42 33, 34<br />
Cable channel 56 56 54, 55 0 44, 45 30, 31, 32,<br />
33, 34<br />
– –<br />
Buried in ground 72, 73 0 70, 71 – 70, 71 0 –<br />
Embedded in structure 57, 58 3 1, 2,<br />
59, 60<br />
50, 51, 52, 53 44, 45 0 – –<br />
Surface mounted – 20, 21 4, 5 6, 7, 8, 9, 12, 13, 14 6, 7, 30, 31, 32, 36 –<br />
22, 23 8, 9 33, 34<br />
Overhead – – 0 10, 11 – 30, 31, 32<br />
33, 34<br />
36 35<br />
Immersed<br />
– Not permitted.<br />
80 80 0 – 0 0 – –<br />
0 Not applicable, or not normally used in practice.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
Fig. G10 : Examples <strong>of</strong> methods <strong>of</strong> <strong>installation</strong> (part <strong>of</strong> table 52-3 <strong>of</strong> IEC 60364-5-52) (continued on next page)<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
Examples <strong>of</strong> wiring systems and reference methods <strong>of</strong><br />
<strong>installation</strong>s<br />
An illustration <strong>of</strong> some <strong>of</strong> the many different wiring systems and methods <strong>of</strong><br />
<strong>installation</strong> is provided in Figure G10.<br />
Several reference methods are defined (with code letters A to G), grouping<br />
<strong>installation</strong> methods having the same characteristics relative to the current-carrying<br />
capacities <strong>of</strong> the wiring systems.<br />
Item No. Methods <strong>of</strong> <strong>installation</strong> Description Reference method <strong>of</strong><br />
<strong>installation</strong> to be used to<br />
obtain current-carrying<br />
capacity<br />
1 Insulated conductors or single-core<br />
cables in conduit in a thermally<br />
insulated wall<br />
A1<br />
Room<br />
2 Multi-core cables in conduit in a A2<br />
thermally insulated wall<br />
4 Insulated conductors or single-core B1<br />
cables in conduit on a wooden, or<br />
masonry wall or spaced less than<br />
0,3 x conduit diameter from it<br />
5 Multi-core cable in conduit on a B2<br />
wooden, or mansonry wall or spaced<br />
less than 0,3 x conduit diameter<br />
from it<br />
20 Single-core or multi-core cables: C<br />
- fixed on, or sapced less than 0.3 x cable<br />
diameter from a wooden wall<br />
30 On unperforated tray C<br />
0.3 De<br />
0.3 De<br />
Room<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G9<br />
© Schneider Electric - all rights reserved
G10<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
Item No. Methods <strong>of</strong> <strong>installation</strong> Description Reference method <strong>of</strong><br />
<strong>installation</strong> to be used to<br />
obtain current-carrying<br />
capacity<br />
31 0.3 De<br />
On perforated tray E or F<br />
0.3 De<br />
36 Bare or insulated conductors on G<br />
insulators<br />
70 Multi-core cables in conduit or in cable D<br />
ducting in the ground<br />
71 Single-core cable in conduit or in cable D<br />
ducting in the ground<br />
Fig. G10 : Examples <strong>of</strong> methods <strong>of</strong> <strong>installation</strong> (part <strong>of</strong> table 52-3 <strong>of</strong> IEC 60364-5-52)<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
Maximum operating temperature:<br />
The current-carrying capacities given in the subsequent tables have been<br />
determined so that the maximum insulation temperature is not exceeded for<br />
sustained periods <strong>of</strong> time.<br />
For different type <strong>of</strong> insulation material, the maximum admissible temperature is<br />
given in Figure G11.<br />
Type <strong>of</strong> insulation Temperature limit °C<br />
Polyvinyl-chloride (PVC) 70 at the conductor<br />
Cross-linked polyethylene (XLPE) and ethylene 90 at the conductor<br />
propylene rubber (EPR)<br />
Mineral (PVC covered or bare exposed to touch) 70 at the sheath<br />
Mineral (bare not exposed to touch and not in 105 at the seath<br />
contact with combustible material)<br />
Fig. G11 : Maximum operating temperatures for types <strong>of</strong> insulation (table 52-4 <strong>of</strong> IEC 60364-5-52)<br />
Correction factors:<br />
In order to take environnement or special conditions <strong>of</strong> <strong>installation</strong> into account,<br />
correction factors have been introduced.<br />
The cross sectional area <strong>of</strong> cables is determined using the rated load current I B<br />
divided by different correction factors, k1, k2, ...:<br />
I<br />
I' B<br />
B =<br />
k1⋅k 2 ...<br />
I’ B is the corrected load current, to be compared to the current-carrying capacity <strong>of</strong><br />
the considered cable.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
b Ambient temperature<br />
The current-carrying capacities <strong>of</strong> cables in the air are based on an average air<br />
temperature equal to 30 °C. For other temperatures, the correction factor is given in<br />
Figure G12 for PVC, EPR and XLPE insulation material.<br />
The related correction factor is here noted k 1.<br />
Ambient temperature °C Insulation<br />
PVC XLPE and EPR<br />
10 1.22 1.15<br />
15 1.17 1.12<br />
20 1.12 1.08<br />
25 1.06 1.04<br />
35 0.94 0.96<br />
40 0.87 0.91<br />
45 0.79 0.87<br />
50 0.71 0.82<br />
55 0.61 0.76<br />
60 0.50 0.71<br />
65 - 0.65<br />
70 - 0.58<br />
75 - 0.50<br />
80 - 0.41<br />
Fig. G12 : Correction factors for ambient air temperatures other than 30 °C to be applied to the<br />
current-carrying capacities for cables in the air (from table A.52-14 <strong>of</strong> IEC 60364-5-52)<br />
The current-carrying capacities <strong>of</strong> cables in the ground are based on an average<br />
ground temperature equal to 20 °C. For other temperatures, the correction factor is<br />
given in Figure G13 for PVC, EPR and XLPE insulation material.<br />
The related correction factor is here noted k 2.<br />
Ground temperature °C Insulation<br />
PVC XLPE and EPR<br />
10 1.10 1.07<br />
15 1.05 1.04<br />
25 0.95 0.96<br />
30 0.89 0.93<br />
35 0.84 0.89<br />
40 0.77 0.85<br />
45 0.71 0.80<br />
50 0.63 0.76<br />
55 0.55 0.71<br />
60 0.45 0.65<br />
65 - 0.60<br />
70 - 0.53<br />
75 - 0.46<br />
80 - 0.38<br />
Fig. G13 : Correction factors for ambient ground temperatures other than 20 °C to be applied to<br />
the current-carrying capacities for cables in ducts in the ground (from table A.52-15 <strong>of</strong><br />
IEC 60364-5-52)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G11<br />
© Schneider Electric - all rights reserved
G12<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
b Soil thermal resistivity<br />
The current-carrying capacities <strong>of</strong> cables in the ground are based on a ground<br />
resistivity equal to 2.5 K.m/W. For other values, the correction factor is given in<br />
Figure G14.<br />
The related correction factor is here noted k3.<br />
Thermal resistivity, K.m/W 1 1.5 2 2.5 3<br />
Correction factor 1.18 1.1 1.05 1 0.96<br />
Fig. G14 : Correction factors for cables in buried ducts for soil thermal resistivities other than 2.5<br />
K.m/W to be applied to the current-carrying capacities for reference method D (table A52.16 <strong>of</strong><br />
IEC 60364-5-52)<br />
Based on experience, a relationship exist between the soil nature and resistivity.<br />
Then, empiric values <strong>of</strong> correction factors k3 are proposed in Figure G15, depending<br />
on the nature <strong>of</strong> soil.<br />
Fig. G15 : Correction factor k3 depending on the nature <strong>of</strong> soil<br />
b Grouping <strong>of</strong> conductors or cables<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Nature <strong>of</strong> soil k3<br />
Very wet soil (saturated) 1.21<br />
Wet soil 1.13<br />
Damp soil 1.05<br />
Dry soil 1.00<br />
Very dry soil (sunbaked) 0.86<br />
The current-carrying capacities given in the subsequent tables relate to single<br />
circuits consisting <strong>of</strong> the following numbers <strong>of</strong> loaded conductors:<br />
v Two insulated conductors or two single-core cables, or one twin-core cable<br />
(applicable to single-phase circuits);<br />
v Three insulated conductors or three single-core cables, or one three-core cable<br />
(applicable to three-phase circuits).<br />
Where more insulated conductors or cables are installed in the same group, a group<br />
reduction factor (here noted k4) shall be applied.<br />
Examples are given in Figures G16 to G18 for different configurations (<strong>installation</strong><br />
methods, in free air or in the ground).<br />
Figure G16 gives the values <strong>of</strong> correction factor k4 for different configurations <strong>of</strong><br />
unburied cables or conductors, grouping <strong>of</strong> more than one circuit or multi-core<br />
cables.<br />
Arrangement Number <strong>of</strong> circuits or multi-core cables Reference methods<br />
(cables touching) 1 2 3 4 5 6 7 8 9 12 16 20<br />
Bunched in air, on a 1.00 0.80 0.70 0.65 0.60 0.57 0.54 0.52 0.50 0.45 0.41 0.38 Methods A to F<br />
surface, embedded or<br />
enclosed<br />
Single layer on wall, floor 1.00 0.85 0.79 0.75 0.73 0.72 0.72 0.71 0.70 No further reduction Method C<br />
or unperforated tray factor for more than<br />
Single layer fixed directly 0.95 0.81 0.72 0.68 0.66 0.64 0.63 0.62 0.61 nine circuits or<br />
under a wooden ceiling multi-core cables<br />
Single layer on a 1.00 0.88 0.82 0.77 0.75 0.73 0.73 0.72 0.72 Methods E and F<br />
perforated horizontal or<br />
vertical tray<br />
Single layer on ladder 1.00 0.87 0.82 0.80 0.80 0.79 0.79 0.78 0.78<br />
support or cleats etc.<br />
Fig. G16 : Reduction factors for groups <strong>of</strong> more than one circuit or <strong>of</strong> more than one multi-core cable (table A.52-17 <strong>of</strong> IEC 60364-5-52)
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
Figure G1 gives the values <strong>of</strong> correction factor k 4 for different configurations <strong>of</strong><br />
unburied cables or conductors, for groups <strong>of</strong> more than one circuit <strong>of</strong> single-core<br />
cables in free air.<br />
Method <strong>of</strong> <strong>installation</strong> Number Number <strong>of</strong> three-phase Use as a<br />
<strong>of</strong> tray circuits multiplier to<br />
rating for<br />
1 2 3<br />
Perforated 31 Touching 1 0.98 0.91 0.87 Three cables in<br />
trays horizontal<br />
2 0.96 0.87 0.81 formation<br />
20 mm<br />
3 0.95 0.85 0.78<br />
Vertical 31 Touching 1 0.96 0.86 Three cables in<br />
perforated vertical<br />
trays<br />
225 mm<br />
2 0.95 0.84 formation<br />
Ladder 32 1 1.00 0.97 0.96 Three cables in<br />
supports, Touching<br />
horizontal<br />
cleats, etc... 33 2 0.98 0.93 0.89 formation<br />
34 3 0.97 0.90 0.86<br />
20 mm<br />
Perforated<br />
trays<br />
31<br />
2De<br />
De 1 1.00 0.98 0.96 Three cables in<br />
trefoil formation<br />
2 0.97 0.93 0.89<br />
20 mm<br />
3 0.96 0.92 0.86<br />
Vertical<br />
perforated<br />
31 De<br />
Spaced 1 1.00 0.91 0.89<br />
trays<br />
225 mm<br />
2 1.00 0.90 0.86<br />
2De<br />
Ladder<br />
supports,<br />
32 2De<br />
De<br />
1 1.00 1.00 1.00<br />
cleats, etc... 33 2 0.97 0.95 0.93<br />
34 20 mm 3 0.96 0.94 0.90<br />
Fig. G17 : Reduction factors for groups <strong>of</strong> more than one circuit <strong>of</strong> single-core cables to be applied to reference rating for one circuit <strong>of</strong> single-core cables in free air<br />
- Method <strong>of</strong> <strong>installation</strong> F. (table A.52.21 <strong>of</strong> IEC 60364-5-52)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G13<br />
© Schneider Electric - all rights reserved
G14<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
Figure G18 gives the values <strong>of</strong> correction factor k4 for different configurations <strong>of</strong><br />
cables or conductors laid directly in the ground.<br />
Number Cable to cable clearance (a) a<br />
<strong>of</strong> circuits Nil (cables One cable 0.125 m 0.25 m 0.5 m<br />
touching) diameter<br />
2 0.75 0.80 0.85 0.90 0.90<br />
3 0.65 0.70 0.75 0.80 0.85<br />
4 0.60 0.60 0.70 0.75 0.80<br />
5 0.55 0.55 0.65 0.70 0.80<br />
6 0.50 0.55 0.60 0.70 0.80<br />
a Multi-core cables<br />
a Single-core cables<br />
Fig. G18 : Reduction factors for more than one circuit, single-core or multi-core cables laid<br />
directly in the ground. Installation method D. (table 52-18 <strong>of</strong> IEC 60364-5-52)<br />
b Harmonic current<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
a a<br />
a a<br />
The current-carrying capacity <strong>of</strong> three-phase, 4-core or 5-core cables is based on<br />
the assumption that only 3 conductors are fully loaded.<br />
However, when harmonic currents are circulating, the neutral current can be<br />
significant, and even higher than the phase currents. This is due to the fact that the<br />
3 rd harmonic currents <strong>of</strong> the three phases do not cancel each other, and sum up in<br />
the neutral conductor.<br />
This <strong>of</strong> course affects the current-carrying capacity <strong>of</strong> the cable, and a correction<br />
factor noted here k5 shall be applied.<br />
In addition, if the 3 rd harmonic percentage h 3 is greater than 33%, the neutral current<br />
is greater than the phase current and the cable size selection is based on the neutral<br />
current. The heating effect <strong>of</strong> harmonic currents in the phase conductors has also to<br />
be taken into account.<br />
The values <strong>of</strong> k5 depending on the 3 rd harmonic content are given in Figure G19.<br />
Third harmonic content Correction factor<br />
<strong>of</strong> phase current % Size selection is based Size selection is based<br />
on phase current on neutral current<br />
0 - 15 1.0<br />
15 - 33 0.86<br />
33 - 45 0.86<br />
> 45 1.0<br />
Fig. G19 : Correction factors for harmonic currents in four-core and five-core cables (table D.52.1<br />
<strong>of</strong> IEC 60364-5-52)<br />
Admissible current as a function <strong>of</strong> nominal cross-sectional<br />
area <strong>of</strong> conductors<br />
IEC standard 60364-5-52 proposes extensive information in the form <strong>of</strong> tables<br />
giving the admissible currents as a function <strong>of</strong> cross-sectional area <strong>of</strong> cables. Many<br />
parameters are taken into account, such as the method <strong>of</strong> <strong>installation</strong>, type <strong>of</strong><br />
insulation material, type <strong>of</strong> conductor material, number <strong>of</strong> loaded conductors.
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
As an example, Figure G20 gives the current-carrying capacities for different<br />
methods <strong>of</strong> <strong>installation</strong> <strong>of</strong> PVC insulation, three loaded copper or aluminium<br />
conductors, free air or in ground.<br />
Nominal Installation methods<br />
cross-sectional A1 A2 B1 B2 C D<br />
area <strong>of</strong> conductors<br />
(mm 2 )<br />
1 2 3 4 5 6<br />
Copper<br />
1.5 13.5 13 15.5 15 17.5 18<br />
2.5 18 17.5 21 20 24 24<br />
4 24 23 28 27 32 31<br />
6 31 29 36 34 41 39<br />
10 42 39 50 46 57 52<br />
16 56 52 68 62 76 67<br />
25 73 68 89 80 96 86<br />
35 89 83 110 99 119 103<br />
50 108 99 134 118 144 122<br />
70 136 125 171 149 184 151<br />
95 164 150 207 179 223 179<br />
120 188 172 239 206 259 203<br />
150 216 196 - - 299 230<br />
185 245 223 - - 341 258<br />
240 286 261 - - 403 297<br />
300 328 298 - - 464 336<br />
Aluminium<br />
2.5 14 13.5 16.5 15.5 18.5 18.5<br />
4 18.5 17.5 22 21 25 24<br />
6 24 23 28 27 32 30<br />
10 32 31 39 36 44 40<br />
16 43 41 53 48 59 52<br />
25 57 53 70 62 73 66<br />
35 70 65 86 77 90 80<br />
50 84 78 104 92 110 94<br />
70 107 98 133 116 140 117<br />
95 129 118 161 139 170 138<br />
120 149 135 186 160 197 157<br />
150 170 155 - - 227 178<br />
185 194 176 - - 259 200<br />
240 227 207 - - 305 230<br />
300 261 237 - - 351 260<br />
Fig. G20 : Current-carrying capacities in amperes for different methods <strong>of</strong> <strong>installation</strong>, PVC insulation, three loaded conductors, copper or aluminium, conductor<br />
temperature: 70 °C, ambient temperature: 30 °C in air, 20 °C in ground (table A.52.4 <strong>of</strong> IEC 60364-5-52)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G15<br />
© Schneider Electric - all rights reserved
G16<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
2.3 Recommended simplified approach for cables<br />
In order to facilitate the selection <strong>of</strong> cables, 2 simplified tables are proposed, for<br />
unburied and buried cables.<br />
These tables summarize the most commonly used configurations and give easier<br />
access to the information.<br />
b Unburied cables:<br />
Fig. G21a : Current-carrying capacity in amperes (table B.52-1 <strong>of</strong> IEC 60364-5-52)<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
Reference Number <strong>of</strong> loaded conductors and type <strong>of</strong> insulation<br />
methods<br />
A1 2 PVC 3 PVC 3 XLPE 2 XLPE<br />
A2 3 PVC 2 PVC 3 XLPE 2 XLPE<br />
B1 3 PVC 2 PVC 3 XLPE 2 XLPE<br />
B2 3 PVC 2 PVC 3 XLPE 2 XLPE<br />
C 3 PVC 2 PVC 3 XLPE 2 XLPE<br />
E 3 PVC 2 PVC 3 XLPE 2 XLPE<br />
F 3 PVC 2 PVC 3 XLPE 2 XLPE<br />
1 2 3 4 5 6 8 9 10 11 12 13<br />
Size (mm 2 )<br />
Copper<br />
1.5 13 13 .5 14.5 15.5 17 18.5 19.5 22 23 24 26 -<br />
2.5 17.5 18 19.5 21 23 25 27 30 31 33 36 -<br />
4 23 24 26 28 31 34 36 40 42 45 49 -<br />
6 29 31 34 36 40 43 46 51 54 58 63 -<br />
10 39 42 46 50 54 60 63 70 75 80 86 -<br />
16 52 56 61 68 73 80 85 94 100 107 115 -<br />
25 68 73 80 89 95 101 110 119 127 135 149 161<br />
35 - - - 110 117 126 137 147 158 169 185 200<br />
50 - - - 134 141 153 167 179 192 207 225 242<br />
70 - - - 171 179 196 213 229 246 268 289 310<br />
95 - - - 207 216 238 258 278 298 328 352 377<br />
120 - - - 239 249 276 299 322 346 382 410 437<br />
150 - - - - 285 318 344 371 395 441 473 504<br />
185 - - - - 324 362 392 424 450 506 542 575<br />
240 - - - - 380 424 461 500 538 599 641 679<br />
Aluminium<br />
2.5 13.5 14 15 16.5 18.5 19.5 21 23 24 26 28 -<br />
4 17.5 18.5 20 22 25 26 28 31 32 35 38 -<br />
6 23 24 26 28 32 33 36 39 42 45 49 -<br />
10 31 32 36 39 44 46 49 54 58 62 67 -<br />
16 41 43 48 53 58 61 66 73 77 84 91 -<br />
25 53 57 63 70 73 78 83 90 97 101 108 121<br />
35 - - - 86 90 96 103 112 120 126 135 150<br />
50 - - - 104 110 117 125 136 146 154 164 184<br />
70 - - - 133 140 150 160 174 187 198 211 237<br />
95 - - - 161 170 183 195 211 227 241 257 289<br />
120 - - - 186 197 212 226 245 263 280 300 337<br />
150 - - - - 226 245 261 283 304 324 346 389<br />
185 - - - - 256 280 298 323 347 371 397 447<br />
240 - - - - 300 330 352 382 409 439 470 530<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
Correction factors are given in Figure G21b for groups <strong>of</strong> several circuits or multicore<br />
cables:<br />
Arrangement Number <strong>of</strong> circuits or multi-core cables<br />
1 2 3 4 6 9 12 16 20<br />
Embedded or enclosed 1.00 0.80 0.70 0.70 0.55 0.50 0.45 0.40 0.40<br />
Single layer on walls, floors 1.00 0.85 0.80 0.75 0.70 0.70 - - -<br />
or on unperforatedtrays<br />
Single layer fixed directly 0.95 0.80 0.70 0.70 0.65 0.60 - - -<br />
under a ceiling<br />
Single layer on perforated 1.00 0.90 0.80 0.75 0.75 0.70 - - -<br />
horizontal trays or on vertical trays<br />
Single layer on cable 1.00 0.85 0.80 0.80 0.80 0.80 - - -<br />
ladder supports or cleats, etc...<br />
Fig. G21b : Reduction factors for groups <strong>of</strong> several circuits or <strong>of</strong> several multi-core cables<br />
(table B.52-3 <strong>of</strong> IEC 60364-5-52)<br />
b Buried cables:<br />
Installation Size Number <strong>of</strong> loaded conductors and type <strong>of</strong> insulation<br />
method mm 2 Two PVC Three PVC Two XLPE Three XLPE<br />
D Copper<br />
1.5 22 18 26 22<br />
2.5 29 24 34 29<br />
4 38 31 44 37<br />
6 47 39 56 46<br />
10 63 52 73 61<br />
16 81 67 95 79<br />
25 104 86 121 101<br />
35 125 103 146 122<br />
50 148 122 173 144<br />
70 183 151 213 178<br />
95 216 179 252 211<br />
120 246 203 287 240<br />
150 278 230 324 271<br />
185 312 258 363 304<br />
240 361 297 419 351<br />
300 408 336 474 396<br />
D Aluminium<br />
2.5 22 18.5 26 22<br />
4 29 24 34 29<br />
6 36 30 42 36<br />
10 48 40 56 47<br />
16 62 52 73 61<br />
25 80 66 93 78<br />
35 96 80 112 94<br />
50 113 94 132 112<br />
70 140 117 163 138<br />
95 166 138 193 164<br />
120 189 157 220 186<br />
150 213 178 249 210<br />
185 240 200 279 236<br />
240 277 230 322 272<br />
300 313 260 364 308<br />
Fig. G22 : Current-carrying capacity in amperes (table B.52-1 <strong>of</strong> IEC 60364-5-52)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G1<br />
© Schneider Electric - all rights reserved
G18<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
2.4 Busbar trunking systems<br />
The selection <strong>of</strong> busbar trunking systems is very straightforward, using the data<br />
provided by the manufacturer. Methods <strong>of</strong> <strong>installation</strong>, insulation materials, correction<br />
factors for grouping are not relevant parameters for this technology.<br />
The cross section area <strong>of</strong> any given model has been determined by the manufacturer<br />
based on:<br />
b The rated current,<br />
b An ambient air temperature equal to 35 °C,<br />
b 3 loaded conductors.<br />
Rated current<br />
The rated current can be calculated taking account <strong>of</strong>:<br />
b The layout,<br />
b The current absorbed by the different loads connected along the trunking system.<br />
Ambient temperature<br />
A correction factor has to be applied for temperature higher than 35 °C. The<br />
correction factor applicable to medium and high power range (up to 4,000 A) is given<br />
in Figure G23a.<br />
°C 35 40 45 50 55<br />
Correction factor 1 0.97 0.93 0.90 0.86<br />
Fig. G23a : Correction factor for air temperature higher than 35 °C<br />
Neutral current<br />
Where 3 rd harmonic currents are circulating, the neutral conductor may be carrying a<br />
significant current and the corresponding additional power losses must be taken into<br />
account.<br />
Figure G23b represents the maximum admissible phase and neutral currents (per<br />
unit) in a high power busbar trunking system as functions <strong>of</strong> 3 rd harmonic level.<br />
Maximum admissible current (p.u)<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 10 20 30 40<br />
Fig. G23b : Maximum admissible currents (p.u.) in a busbar trunking system as functions <strong>of</strong> the<br />
3 rd harmonic level.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
50 60 70<br />
3 rd harmonic current level (%)<br />
Neutral conductor<br />
Phase conductor<br />
80 90
G - Sizing and protection <strong>of</strong> conductors<br />
2 Practical method for determining<br />
the smallest allowable crosssectional<br />
area <strong>of</strong> circuit conductors<br />
The layout <strong>of</strong> the trunking system depends on the position <strong>of</strong> the current consumers,<br />
the location <strong>of</strong> the power source and the possibilities for fixing the system.<br />
v One single distribution line serves a 4 to 6 meter area<br />
v Protection devices for current consumers are placed in tap-<strong>of</strong>f units, connected<br />
directly to usage points.<br />
v One single feeder supplies all current consumers <strong>of</strong> different powers.<br />
Once the trunking system layout is established, it is possible to calculate the<br />
absorbed current I n on the distribution line.<br />
I n is equal to the sum <strong>of</strong> absorbed currents by the current I n consumers: I n = Σ I B.<br />
The current consumers do not all work at the same time and are not permanently on<br />
full load, so we have to use a clustering coefficient k S : I n = Σ (I B . k S).<br />
Application Number <strong>of</strong> current consumers Ks Coefficient<br />
Lighting, Heating 1<br />
Distribution (engineering<br />
workshop)<br />
2...3<br />
4...5<br />
6...9<br />
10...40<br />
40 and over<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
0.9<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
Note : for industrial <strong>installation</strong>s, remember to take account <strong>of</strong> upgrading <strong>of</strong> the machine<br />
equipment base. As for a switchboard, a 20 % margin is recommended:<br />
I n ≤ I B x k s x 1.2.<br />
Fig G24 : Clustering coefficient according to the number <strong>of</strong> current consumers<br />
G19<br />
© Schneider Electric - all rights reserved
G20<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
3 Determination <strong>of</strong> voltage drop<br />
The impedance <strong>of</strong> circuit conductors is low but not negligible: when carrying<br />
load current there is a voltage drop between the origin <strong>of</strong> the circuit and the load<br />
terminals. The correct operation <strong>of</strong> a load (a motor, lighting circuit, etc.) depends<br />
on the voltage at its terminals being maintained at a value close to its rated value.<br />
It is necessary therefore to determine the circuit conductors such that at full-load<br />
current, the load terminal voltage is maintained within the limits required for correct<br />
performance.<br />
This section deals with methods <strong>of</strong> determining voltage drops, in order to check that:<br />
b They comply with the particular standards and regulations in force<br />
b They can be tolerated by the load<br />
b They satisfy the essential operational requirements<br />
3.1 Maximum voltage drop<br />
Maximum allowable voltage-drop vary from one country to another. Typical values for<br />
LV <strong>installation</strong>s are given below in Figure G25.<br />
Type <strong>of</strong> <strong>installation</strong>s Lighting Other uses<br />
circuits (heating and power)<br />
A low-voltage service connection from 3% 5%<br />
a LV public power distribution network<br />
Consumers MV/LV substation supplied 6% 8%<br />
from a public distribution MV system<br />
Fig. G25 : Maximum voltage-drop between the service-connection point and the point <strong>of</strong> utilization<br />
These voltage-drop limits refer to normal steady-state operating conditions and do<br />
not apply at times <strong>of</strong> motor starting, simultaneous switching (by chance) <strong>of</strong> several<br />
loads, etc. as mentioned in <strong>Chapter</strong> A Sub-clause 4.3 (factor <strong>of</strong> simultaneity, etc.).<br />
When voltage drops exceed the values shown in Figure G25, larger cables (wires)<br />
must be used to correct the condition.<br />
The value <strong>of</strong> 8%, while permitted, can lead to problems for motor loads; for example:<br />
b In general, satisfactory motor performance requires a voltage within ± 5% <strong>of</strong> its<br />
rated nominal value in steady-state operation,<br />
b Starting current <strong>of</strong> a motor can be 5 to 7 times its full-load value (or even higher).<br />
If an 8% voltage drop occurs at full-load current, then a drop <strong>of</strong> 40% or more will<br />
occur during start-up. In such conditions the motor will either:<br />
v Stall (i.e. remain stationary due to insufficient torque to overcome the load torque)<br />
with consequent over-heating and eventual trip-out<br />
v Or accelerate very slowly, so that the heavy current loading (with possibly<br />
undesirable low-voltage effects on other equipment) will continue beyond the normal<br />
start-up period<br />
b Finally an 8% voltage drop represents a continuous power loss, which, for continuous<br />
loads will be a significant waste <strong>of</strong> (metered) energy. For these reasons it is<br />
recommended that the maximum value <strong>of</strong> 8% in steady operating conditions should not<br />
be reached on circuits which are sensitive to under-voltage problems (see Fig. G26).<br />
Load<br />
Fig. G26 : Maximum voltage drop<br />
LV consumer<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
5% (1)<br />
MV consumer<br />
8% (1)<br />
(1) Between the LV supply point and the load
G - Sizing and protection <strong>of</strong> conductors<br />
3 Determination <strong>of</strong> voltage drop<br />
3.2 Calculation <strong>of</strong> voltage drop in steady load<br />
conditions<br />
Use <strong>of</strong> formulae<br />
Figure G27 below gives formulae commonly used to calculate voltage drop in a<br />
given circuit per kilometre <strong>of</strong> length.<br />
If:<br />
b IB: The full load current in amps<br />
b L: Length <strong>of</strong> the cable in kilometres<br />
b R: Resistance <strong>of</strong> the cable conductor in Ω/km<br />
2<br />
22.5 Ω mm / km<br />
R =<br />
for copper<br />
2<br />
S c.s.a. in mm<br />
( )<br />
2<br />
36 Ω mm / km<br />
R =<br />
for aluminium<br />
2<br />
S c.s.a. in mm<br />
( )<br />
Note: R is negligible above a c.s.a. <strong>of</strong> 500 mm 2<br />
b X: inductive reactance <strong>of</strong> a conductor in Ω/km<br />
Note: X is negligible for conductors <strong>of</strong> c.s.a. less than 50 mm 2 . In the absence <strong>of</strong> any<br />
other information, take X as being equal to 0.08 Ω/km.<br />
b ϕ: phase angle between voltage and current in the circuit considered, generally:<br />
v Incandescent lighting: cos ϕ = 1<br />
v Motor power:<br />
- At start-up: cos ϕ = 0.35<br />
- In normal service: cos ϕ = 0.8<br />
b Un: phase-to-phase voltage<br />
b Vn: phase-to-neutral voltage<br />
For prefabricated pre-wired ducts and bustrunking, resistance and inductive<br />
reactance values are given by the manufacturer.<br />
Circuit Voltage drop (ΔU)<br />
in volts in %<br />
Single phase: phase/phase ∆U = 2I B R cos ϕ + X sin ϕ L<br />
Fig. G27 : Voltage-drop formulae<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
( )<br />
100 ∆U<br />
Un<br />
Single phase: phase/neutral ∆U = 2I B( R cos ϕ + X sin ϕ) L<br />
100 ∆ U<br />
Vn<br />
Balanced 3-phase: 3 phases ∆U = 3 I B( R cos ϕ + X sin ϕ) L 100 ∆ U<br />
Un<br />
(with or without neutral)<br />
Simplified table<br />
Calculations may be avoided by using Figure G28 next page, which gives, with<br />
an adequate approximation, the phase-to-phase voltage drop per km <strong>of</strong> cable per<br />
ampere, in terms <strong>of</strong>:<br />
b Kinds <strong>of</strong> circuit use: motor circuits with cos ϕ close to 0.8, or lighting with a cos ϕ<br />
close to 1.<br />
b Type <strong>of</strong> cable; single-phase or 3-phase<br />
Voltage drop in a cable is then given by:<br />
K x IB x L<br />
K is given by the table,<br />
IB is the full-load current in amps,<br />
L is the length <strong>of</strong> cable in km.<br />
The column motor power “cos ϕ = 0.35” <strong>of</strong> Figure G28 may be used to compute the<br />
voltage drop occurring during the start-up period <strong>of</strong> a motor (see example no. 1 after<br />
the Figure G28).<br />
G21<br />
© Schneider Electric - all rights reserved
G22<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
c.s.a. in mm 2 Single-phase circuit Balanced three-phase circuit<br />
Motor power Lighting Motor power Lighting<br />
Normal service Start-up Normal service Start-up<br />
Cu Al cos ϕ = 0.8 cos ϕ = 0.35 cos ϕ = 1 cos ϕ = 0.8 cos ϕ = 0.35 cos ϕ = 1<br />
1.5 24 10.6 30 20 9.4 25<br />
2.5 14.4 6.4 18 12 5.7 15<br />
4 9.1 4.1 11.2 8 3.6 9.5<br />
6 10 6.1 2.9 7.5 5.3 2.5 6.2<br />
10 16 3.7 1.7 4.5 3.2 1.5 3.6<br />
16 25 2.36 1.15 2.8 2.05 1 2.4<br />
25 35 1.5 0.75 1.8 1.3 0.65 1.5<br />
35 50 1.15 0.6 1.29 1 0.52 1.1<br />
50 70 0.86 0.47 0.95 0.75 0.41 0.77<br />
70 120 0.64 0.37 0.64 0.56 0.32 0.55<br />
95 150 0.48 0.30 0.47 0.42 0.26 0.4<br />
120 185 0.39 0.26 0.37 0.34 0.23 0.31<br />
150 240 0.33 0.24 0.30 0.29 0.21 0.27<br />
185 300 0.29 0.22 0.24 0.25 0.19 0.2<br />
240 400 0.24 0.2 0.19 0.21 0.17 0.16<br />
300 500 0.21 0.19 0.15 0.18 0.16 0.13<br />
Fig. G28 : Phase-to-phase voltage drop ΔU for a circuit, in volts per ampere per km<br />
1,000 A<br />
400 V<br />
Fig. G29 : Example 1<br />
50 m / 35 mm 2 Cu<br />
IB = 100 A<br />
(500 A during start-up)<br />
3 Determination <strong>of</strong> voltage drop<br />
Examples<br />
Example 1 (see Fig. G29)<br />
A three-phase 35 mm 2 copper cable 50 metres long supplies a 400 V motor taking:<br />
b 100 A at a cos ϕ = 0.8 on normal permanent load<br />
b 500 A (5 In) at a cos ϕ = 0.35 during start-up<br />
The voltage drop at the origin <strong>of</strong> the motor cable in normal circumstances (i.e. with<br />
the distribution board <strong>of</strong> Figure G29 distributing a total <strong>of</strong> 1,000 A) is 10 V phase-tophase.<br />
What is the voltage drop at the motor terminals:<br />
b In normal service?<br />
b During start-up?<br />
Solution:<br />
b Voltage drop in normal service conditions:<br />
∆<br />
∆U%<br />
100 U<br />
=<br />
Un<br />
Table G28 shows 1 V/A/km so that:<br />
ΔU for the cable = 1 x 100 x 0.05 = 5 V<br />
ΔU total = 10 + 5 = 15 V = i.e.<br />
15<br />
x 100 3.75%<br />
400 =<br />
This value is less than that authorized (8%) and is satisfactory.<br />
b Voltage drop during motor start-up:<br />
ΔUcable = 0.52 x 500 x 0.05 = 13 V<br />
Owing to the additional current taken by the motor when starting, the voltage drop at<br />
the distribution board will exceed 10 Volts.<br />
Supposing that the infeed to the distribution board during motor starting is<br />
900 + 500 = 1,400 A then the voltage drop at the distribution board will increase<br />
approximately pro rata, i.e.<br />
10 x 1,400<br />
= 14 V<br />
1,000<br />
ΔU distribution board = 14 V<br />
ΔU for the motor cable = 13 V<br />
ΔU total = 13 + 14 = 27 V i.e.<br />
27<br />
x 100 6.75%<br />
400 =<br />
a value which is satisfactory during motor starting.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
3 Determination <strong>of</strong> voltage drop<br />
Example 2 (see Fig. G30)<br />
A 3-phase 4-wire copper line <strong>of</strong> 70 mm 2 c.s.a. and a length <strong>of</strong> 50 m passes a current<br />
<strong>of</strong> 150 A. The line supplies, among other loads, 3 single-phase lighting circuits, each<br />
<strong>of</strong> 2.5 mm 2 c.s.a. copper 20 m long, and each passing 20 A.<br />
It is assumed that the currents in the 70 mm 2 line are balanced and that the three<br />
lighting circuits are all connected to it at the same point.<br />
What is the voltage drop at the end <strong>of</strong> the lighting circuits?<br />
Solution:<br />
b Voltage drop in the 4-wire line:<br />
∆<br />
∆U%<br />
100 U<br />
=<br />
Un<br />
Figure G28 shows 0.55 V/A/km<br />
ΔU line = 0.55 x 150 x 0.05 = 4.125 V phase-to-phase<br />
4 . 125<br />
which gives: = 2.38 V phase to neutral.<br />
3<br />
b Voltage drop in any one <strong>of</strong> the lighting single-phase circuits:<br />
ΔU for a single-phase circuit = 18 x 20 x 0.02 = 7.2 V<br />
The total voltage drop is therefore<br />
7.2 + 2.38 = 9.6 V<br />
9.6 V<br />
x 100 = 4.2%<br />
230 V<br />
This value is satisfactory, being less than the maximum permitted voltage drop <strong>of</strong> 6%.<br />
Fig. G30 : Example 2<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
50 m / 70 mm2 Cu<br />
IB = 150 A<br />
20 m / 2.5 mm2 Cu<br />
IB = 20 A<br />
G23<br />
© Schneider Electric - all rights reserved
G24<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
Knowing the levels <strong>of</strong> 3-phase symmetrical<br />
short-circuit currents (Isc) at different points<br />
in an <strong>installation</strong> is an essential feature <strong>of</strong> its<br />
<strong>design</strong><br />
Isc1<br />
Isc2<br />
Fig. G32 : Case <strong>of</strong> several transformers in parallel<br />
Isc3<br />
Isc1 + Isc2 + Isc3<br />
4 Short-circuit current<br />
A knowledge <strong>of</strong> 3-phase symmetrical short-circuit current values (Isc) at strategic<br />
points <strong>of</strong> an <strong>installation</strong> is necessary in order to determine switchgear (fault current<br />
rating), cables (thermal withstand rating), protective devices (discriminative trip<br />
settings) and so on...<br />
In the following notes a 3-phase short-circuit <strong>of</strong> zero impedance (the so-called bolted<br />
short-circuit) fed through a typical MV/LV distribution transformer will be examined.<br />
Except in very unusual circumstances, this type <strong>of</strong> fault is the most severe, and is<br />
certainly the simplest to calculate.<br />
Short-circuit currents occurring in a network supplied from a generator and also in<br />
DC systems are dealt with in <strong>Chapter</strong> N.<br />
The simplified calculations and practical <strong>rules</strong> which follow give conservative results<br />
<strong>of</strong> sufficient accuracy, in the large majority <strong>of</strong> cases, for <strong>installation</strong> <strong>design</strong> purposes.<br />
4.1 Short-circuit current at the secondary terminals<br />
<strong>of</strong> a MV/LV distribution transformer<br />
The case <strong>of</strong> one transformer<br />
b In a simplified approach, the impedance <strong>of</strong> the MV system is assumed to be<br />
In<br />
negligibly small, so that: Isc<br />
In<br />
Usc<br />
P<br />
3<br />
x 100 x 10<br />
= where = and:<br />
U20<br />
3<br />
P = kVA rating <strong>of</strong> the transformer<br />
U20 = phase-to-phase secondary volts on open circuit<br />
In = nominal current in amps<br />
Isc = short-circuit fault current in amps<br />
Usc = short-circuit impedance voltage <strong>of</strong> the transformer in %.<br />
Typical values <strong>of</strong> Usc for distribution transformers are given in Figure G31.<br />
Fig. G31 : Typical values <strong>of</strong> Usc for different kVA ratings <strong>of</strong> transformers with MV windings y 20 kV<br />
b Example<br />
Transformer rating Usc in %<br />
(kVA) Oil-immersed Cast-resin<br />
dry type<br />
50 to 750 4 6<br />
800 to 3,200 6 6<br />
400 kVA transformer, 420 V at no load<br />
Usc = 4%<br />
400<br />
x 10<br />
55 0 x 100<br />
In = = 550 A I sc = = 13. 7 kA<br />
420 x 3<br />
4<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
3<br />
The case <strong>of</strong> several transformers in parallel feeding a busbar<br />
The value <strong>of</strong> fault current on an outgoing circuit immediately downstream <strong>of</strong><br />
the busbars (see Fig. G32) can be estimated as the sum <strong>of</strong> the Isc from each<br />
transformer calculated separately.<br />
It is assumed that all transformers are supplied from the same MV network, in which<br />
case the values obtained from Figure G31 when added together will give a slightly<br />
higher fault-level value than would actually occur.<br />
Other factors which have not been taken into account are the impedance <strong>of</strong> the<br />
busbars and <strong>of</strong> the circuit-breakers.<br />
The conservative fault-current value obtained however, is sufficiently accurate for<br />
basic <strong>installation</strong> <strong>design</strong> purposes. The choice <strong>of</strong> circuit-breakers and incorporated<br />
protective devices against short-circuit fault currents is described in <strong>Chapter</strong> H Subclause<br />
4.4.
G - Sizing and protection <strong>of</strong> conductors<br />
Fig. G33 : Impedance diagram<br />
Z<br />
(1) Short-circuit MVA: 3 E L Isc where:<br />
b E L = phase-to-phase nominal system voltage expressed in<br />
kV (r.m.s.)<br />
b Isc = 3-phase short-circuit current expressed in kA (r.m.s.)<br />
(2) up to 36 kV<br />
R<br />
X<br />
4 Short-circuit current<br />
4.2 3-phase short-circuit current (Isc) at any point<br />
within a LV <strong>installation</strong><br />
In a 3-phase <strong>installation</strong> Isc at any point is given by:<br />
U<br />
Isc = 20 where<br />
3 ZT<br />
U 20 = phase-to-phase voltage <strong>of</strong> the open circuited secondary windings <strong>of</strong> the power<br />
supply transformer(s).<br />
Z T = total impedance per phase <strong>of</strong> the <strong>installation</strong> upstream <strong>of</strong> the fault location (in Ω)<br />
Method <strong>of</strong> calculating Z T<br />
Each component <strong>of</strong> an <strong>installation</strong> (MV network, transformer, cable, circuit-breaker,<br />
busbar, and so on...) is characterized by its impedance Z, comprising an element<br />
<strong>of</strong> resistance (R) and an inductive reactance (X). It may be noted that capacitive<br />
reactances are not important in short-circuit current calculations.<br />
The parameters R, X and Z are expressed in ohms, and are related by the sides <strong>of</strong> a<br />
right angled triangle, as shown in the impedance diagram <strong>of</strong> Figure G33.<br />
The method consists in dividing the network into convenient sections, and to<br />
calculate the R and X values for each.<br />
Where sections are connected in series in the network, all the resistive elements in<br />
the section are added arithmetically; likewise for the reactances, to give R T and X T.<br />
The impedance (Z T) for the combined sections concerned is then calculated from<br />
2<br />
Z = R + X 2<br />
T T T<br />
Any two sections <strong>of</strong> the network which are connected in parallel, can, if<br />
predominantly both resistive (or both inductive) be combined to give a single<br />
equivalent resistance (or reactance) as follows:<br />
Let R1 and R2 be the two resistances connected in parallel, then the equivalent<br />
resistance R3 will be given by:<br />
R1<br />
x R2<br />
X1<br />
x X2<br />
R3<br />
= or for reactances X3<br />
=<br />
R 1 + R2<br />
X 1 + X2<br />
It should be noted that the calculation <strong>of</strong> X3 concerns only separated circuit without<br />
mutual inductance. If the circuits in parallel are close togother the value <strong>of</strong> X3 will be<br />
notably higher.<br />
Determination <strong>of</strong> the impedance <strong>of</strong> each component<br />
b Network upstream <strong>of</strong> the MV/LV transformer (see Fig. G34)<br />
The 3-phase short-circuit fault level P SC, in kA or in MVA (1) is given by the power<br />
supply authority concerned, from which an equivalent impedance can be deduced.<br />
Psc Uo (V) Ra (mΩ) Xa (mΩ)<br />
250 MVA 420 0.07 0.7<br />
500 MVA 420 0.035 0.351<br />
Fig. G34 : The impedance <strong>of</strong> the MV network referred to the LV side <strong>of</strong> the MV/LV transformer<br />
A formula which makes this deduction and at the same time converts the impedance<br />
to an equivalent value at LV is given, as follows:<br />
Zs U<br />
=<br />
Psc<br />
02<br />
where<br />
Zs = impedance <strong>of</strong> the MV voltage network, expessed in milli-ohms<br />
Uo = phase-to-phase no-load LV voltage, expressed in volts<br />
Psc = MV 3-phase short-circuit fault level, expressed in kVA<br />
The upstream (MV) resistance Ra is generally found to be negligible compared with<br />
the corresponding Xa, the latter then being taken as the ohmic value for Za. If more<br />
accurate calculations are necessary, Xa may be taken to be equal to 0.995 Za and<br />
Ra equal to 0.1 Xa.<br />
Figure G36 gives values for Ra and Xa corresponding to the most common MV (2)<br />
short-circuit levels in utility power-supply networks, namely, 250 MVA and 500 MVA.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G25<br />
© Schneider Electric - all rights reserved
G26<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
(1) For 50 Hz systems, but 0.18 mΩ/m length at 60 Hz<br />
4 Short-circuit current<br />
b Transformers (see Fig. G35)<br />
The impedance Ztr <strong>of</strong> a transformer, viewed from the LV terminals, is given by the<br />
formula:<br />
Ztr U Usc<br />
= x<br />
Pn 202<br />
100<br />
where:<br />
U 20 = open-circuit secondary phase-to-phase voltage expressed in volts<br />
Pn = rating <strong>of</strong> the transformer (in kVA)<br />
Usc = the short-circuit impedance voltage <strong>of</strong> the transformer expressed in %<br />
The transformer windings resistance Rtr can be derived from the total losses as<br />
follows:<br />
2<br />
Pcu x 10<br />
Pcu = 3I<br />
n x Rtr so that Rtr =<br />
2<br />
3I<br />
n<br />
3<br />
in milli-ohms<br />
where<br />
Pcu = total losses in watts<br />
In = nominal full-load current in amps<br />
Rtr = resistance <strong>of</strong> one phase <strong>of</strong> the transformer in milli-ohms (the LV and<br />
corresponding MV winding for one LV phase are included in this resistance value).<br />
2 2<br />
Xtr = Ztr −Rtr<br />
Rated Oil-immersed Cast-resin<br />
For an approximate calculation Rtr may be ignored since X ≈ Z in standard<br />
distribution type transformers.<br />
Power Usc (%) Rtr (mΩ) Xtr (mΩ) Ztr (mΩ) Usc (%) Rtr (mΩ) Xtr (mΩ) Ztr (mΩ)<br />
(kVA)<br />
100 4 37.9 59.5 70.6 6 37.0 99.1 105.8<br />
160 4 16.2 41.0 44.1 6 18.6 63.5 66.2<br />
200 4 11.9 33.2 35.3 6 14.1 51.0 52.9<br />
250 4 9.2 26.7 28.2 6 10.7 41.0 42.3<br />
315 4 6.2 21.5 22.4 6 8.0 32.6 33.6<br />
400 4 5.1 16.9 17.6 6 6.1 25.8 26.5<br />
500 4 3.8 13.6 14.1 6 4.6 20.7 21.2<br />
630 4 2.9 10.8 11.2 6 3.5 16.4 16.8<br />
800 6 2.9 12.9 13.2 6 2.6 13.0 13.2<br />
1,000 6 2.3 10.3 10.6 6 1.9 10.4 10.6<br />
1,250 6 1.8 8.3 8.5 6 1.5 8.3 8.5<br />
1,600 6 1.4 6.5 6.6 6 1.1 6.5 6.6<br />
2,000 6 1.1 5.2 5.3 6 0.9 5.2 5.3<br />
Fig. G35 : Resistance, reactance and impedance values for typical distribution 400 V transformers with MV windings y 20 kV<br />
b Circuit-breakers<br />
In LV circuits, the impedance <strong>of</strong> circuit-breakers upstream <strong>of</strong> the fault location must<br />
be taken into account. The reactance value conventionally assumed is 0.15 mΩ per<br />
CB, while the resistance is neglected.<br />
b Busbars<br />
The resistance <strong>of</strong> busbars is generally negligible, so that the impedance is practically<br />
all reactive, and amounts to approximately 0.15 mΩ/metre (1) length for LV busbars<br />
(doubling the spacing between the bars increases the reactance by about 10% only).<br />
b Circuit conductors<br />
L<br />
The resistance <strong>of</strong> a conductor is given by the formula: Rc = ρ<br />
S<br />
where<br />
ρ = the resistivity constant <strong>of</strong> the conductor material at the normal operating<br />
temperature being:<br />
v 22.5 mΩ.mm 2 /m for copper<br />
v 36 mΩ.mm 2 /m for aluminium<br />
L = length <strong>of</strong> the conductor in m<br />
S = c.s.a. <strong>of</strong> conductor in mm 2<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
Cable reactance values can be obtained from the manufacturers. For c.s.a. <strong>of</strong> less<br />
than 50 mm 2 reactance may be ignored. In the absence <strong>of</strong> other information, a value<br />
<strong>of</strong> 0.08 mΩ/metre may be used (for 50 Hz systems) or 0.096 mΩ/metre (for 60 Hz<br />
systems). For prefabricated bus-trunking and similar pre-wired ducting systems, the<br />
manufacturer should be consulted.<br />
b Motors<br />
At the instant <strong>of</strong> short-circuit, a running motor will act (for a brief period) as a<br />
generator, and feed current into the fault.<br />
In general, this fault-current contribution may be ignored. However, for more precise<br />
calculation, particularly in the case <strong>of</strong> large motors and/or numerous smaller motors,<br />
the total contribution can be estimated from the formula:<br />
Iscm = 3.5 In from each motor i.e. 3.5mIn for m similar motors operating concurrently.<br />
The motors concerned will be the 3-phase motors only; single-phase-motor<br />
contribution being insignificant.<br />
b Fault-arc resistance<br />
Short-circuit faults generally form an arc which has the properties <strong>of</strong> a resistance.<br />
The resistance is not stable and its average value is low, but at low voltage this<br />
resistance is sufficient to reduce the fault-current to some extent. Experience has<br />
shown that a reduction <strong>of</strong> the order <strong>of</strong> 20% may be expected. This phenomenon will<br />
effectively ease the current-breaking duty <strong>of</strong> a CB, but affords no relief for its faultcurrent<br />
making duty.<br />
b Recapitulation table (see Fig. G36)<br />
Parts <strong>of</strong> power-supply system R (mΩ) X (mΩ)<br />
M<br />
Supply network<br />
Figure G34<br />
Transformer<br />
Figure G35<br />
Rtr is <strong>of</strong>ten negligible compared to Xtr<br />
for transformers > 100 kVA<br />
with Ztr<br />
Circuit-breaker Negligible XD = 0.15 mΩ/pole<br />
U Usc<br />
= x<br />
Pn 202<br />
100<br />
Busbars Negligible for S > 200 mm2 in the formula: XB = 0.15 mΩ/m<br />
(1)<br />
Circuit conductors (2) (1) L<br />
R = ρ<br />
S<br />
L<br />
R = ρ<br />
S<br />
Cables: Xc = 0.08 mΩ/m<br />
Motors See Sub-clause 4.2 Motors<br />
(<strong>of</strong>ten negligible at LV)<br />
Three-phase short<br />
circuit current in kA Isc =<br />
3<br />
U20<br />
2 2<br />
RT + XT<br />
U 20: Phase-to-phase no-load secondary voltage <strong>of</strong> MV/LV transformer (in volts).<br />
Psc: 3-phase short-circuit power at MV terminals <strong>of</strong> the MV/LV transformers (in kVA).<br />
Pcu: 3-phase total losses <strong>of</strong> the MV/LV transformer (in watts).<br />
Pn: Rating <strong>of</strong> the MV/LV transformer (in kVA).<br />
Usc: Short-circuit impedance voltage <strong>of</strong> the MV/LV transfomer (in %).<br />
R T : Total resistance. X T: Total reactance<br />
(1) ρ = resistivity at normal temperature <strong>of</strong> conductors in service<br />
b ρ = 22.5 mΩ x mm 2 /m for copper<br />
b ρ = 36 mΩ x mm 2 /m for aluminium<br />
(2) If there are several conductors in parallel per phase, then divide the resistance <strong>of</strong> one conductor by the number <strong>of</strong><br />
conductors. The reactance remains practically unchanged.<br />
Fig. G36 : Recapitulation table <strong>of</strong> impedances for different parts <strong>of</strong> a power-supply system<br />
4 Short-circuit current<br />
Pcu<br />
=<br />
3 n<br />
2<br />
x 10<br />
Rtr<br />
I<br />
3<br />
Ra<br />
= 0.1<br />
Xa<br />
U<br />
Xa = 0 995 Za Za = 20<br />
Psc<br />
2 2<br />
Ztr −Rtr<br />
2<br />
. ;<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G27<br />
© Schneider Electric - all rights reserved
G28<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
LV <strong>installation</strong> R (mΩ) X (mΩ) RT (mΩ) XT (mΩ)<br />
MV network<br />
Psc = 500 MVA<br />
0.035 0.351<br />
Transformer<br />
20 kV/420 V<br />
Pn = 1000 kVA<br />
Usc = 5%<br />
Pcu = 13.3 x 10<br />
2.24 8.10<br />
3 watts<br />
Single-core cables<br />
5 m copper<br />
4 x 240 mm<br />
Xc = 0.08 x 5 = 0.40 2.41 8.85 Isc1 = 26 kA<br />
2 /phase<br />
Main<br />
circuit-breaker<br />
RD = 0 XD = 0.15<br />
Busbars<br />
10 m<br />
RB = 0 XB = 1.5 2.41 10.5 Isc2 = 22 kA<br />
Three-core cable<br />
100 m<br />
95 mm<br />
Xc = 100 x 0.08 = 8 26.1 18.5 Isc3 = 7.4 kA<br />
2 copper<br />
Three-core cable<br />
20 m<br />
10 mm<br />
Xc = 20 x 0.08 = 1.6 71.1 20.1 Isc4 = 3.2 kA<br />
2 22.5 5<br />
Rc = x = 0.12<br />
4 240<br />
100<br />
Rc = 22.5 x = 2 3.68<br />
95<br />
copper<br />
final circuits<br />
Rc = 22.5 x =<br />
20<br />
45<br />
10<br />
Fig. G37 : Example <strong>of</strong> short-circuit current calculations for a LV <strong>installation</strong> supplied at 400 V (nominal) from a 1,000 kVA MV/LV transformer<br />
400 V<br />
47,5 mm 2 , Cu<br />
20 m<br />
IB = 55 A IB = 160 A<br />
Isc = 28 kA<br />
Isc = ?<br />
Fig. G38 : Determination <strong>of</strong> downstream short-circuit current<br />
level Isc using Figure G39<br />
4 Short-circuit current<br />
b Example <strong>of</strong> short-circuit calculations (see Fig. G37)<br />
4.3 Isc at the receiving end <strong>of</strong> a feeder as a function<br />
<strong>of</strong> the Isc at its sending end<br />
The network shown in Figure G38 typifies a case for the application <strong>of</strong> Figure G39<br />
next page, derived by the «method <strong>of</strong> composition» (mentioned in <strong>Chapter</strong> F Subclause<br />
6.2). These tables give a rapid and sufficiently accurate value <strong>of</strong> short-circuit<br />
current at a point in a network, knowing:<br />
b The value <strong>of</strong> short-circuit current upstream <strong>of</strong> the point considered<br />
b The length and composition <strong>of</strong> the circuit between the point at which the shortcircuit<br />
current level is known, and the point at which the level is to be determined<br />
It is then sufficient to select a circuit-breaker with an appropriate short-circuit fault<br />
rating immediately above that indicated in the tables.<br />
If more precise values are required, it is possible to make a detailled calculation<br />
(see Sub-Clause 4.2) or to use a s<strong>of</strong>tware package, such as Ecodial. In such a case,<br />
moreover, the possibility <strong>of</strong> using the cascading technique should be considered,<br />
in which the use <strong>of</strong> a current limiting circuit-breaker at the upstream position would<br />
allow all circuit-breakers downstream <strong>of</strong> the limiter to have a short-circuit current<br />
rating much lower than would otherwise be necessary (See chapter H Sub-Clause 4.5).<br />
Method<br />
Select the c.s.a. <strong>of</strong> the conductor in the column for copper conductors (in this<br />
example the c.s.a. is 47.5 mm 2 ).<br />
Search along the row corresponding to 47.5 mm 2 for the length <strong>of</strong> conductor equal<br />
to that <strong>of</strong> the circuit concerned (or the nearest possible on the low side). Descend<br />
vertically the column in which the length is located, and stop at a row in the middle<br />
section (<strong>of</strong> the 3 sections <strong>of</strong> the Figure) corresponding to the known fault-current<br />
level (or the nearest to it on the high side).<br />
In this case 30 kA is the nearest to 28 kA on the high side. The value <strong>of</strong> short-circuit<br />
current at the downstream end <strong>of</strong> the 20 metre circuit is given at the intersection <strong>of</strong><br />
the vertical column in which the length is located, and the horizontal row corresponding<br />
to the upstream Isc (or nearest to it on the high side).<br />
This value in the example is seen to be 14.7 kA.<br />
The procedure for aluminium conductors is similar, but the vertical column must be<br />
ascended into the middle section <strong>of</strong> the table.<br />
In consequence, a DIN-rail-mounted circuit-breaker rated at 63 A and Isc <strong>of</strong> 25 kA<br />
(such as a NG 125N unit) can be used for the 55 A circuit in Figure G38.<br />
A Compact rated at 160 A with an Isc capacity <strong>of</strong> 25 kA (such as a NS160 unit) can<br />
be used to protect the 160 A circuit.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Isc =<br />
420<br />
2 2<br />
3 RT + XT
G - Sizing and protection <strong>of</strong> conductors<br />
4 Short-circuit current<br />
Copper 230 V / 400 V<br />
c.s.a. <strong>of</strong> phase Length <strong>of</strong> circuit (in metres)<br />
conductors (mm 2 )<br />
1.5 1.3 1.8 2.6 3.6 5.2 7.3 10.3 14.6 21<br />
2.5 1.1 1.5 2.1 3.0 4.3 6.1 8.6 12.1 17.2 24 34<br />
4 1.2 1.7 2.4 3.4 4.9 6.9 9.7 13.7 19.4 27 39 55<br />
6 1.8 2.6 3.6 5.2 7.3 10.3 14.6 21 29 41 58 82<br />
10 2.2 3.0 4.3 6.1 8.6 12.2 17.2 24 34 49 69 97 137<br />
16 1.7 2.4 3.4 4.9 6.9 9.7 13.8 19.4 27 39 55 78 110 155 220<br />
25 1.3 1.9 2.7 3.8 5.4 7.6 10.8 15.2 21 30 43 61 86 121 172 243 343<br />
35 1.9 2.7 3.8 5.3 7.5 10.6 15.1 21 30 43 60 85 120 170 240 340 480<br />
47.5 1.8 2.6 3.6 5.1 7.2 10.2 14.4 20 29 41 58 82 115 163 231 326 461<br />
70 2.7 3.8 5.3 7.5 10.7 15.1 21 30 43 60 85 120 170 240 340<br />
95 2.6 3.6 5.1 7.2 10.2 14.5 20 29 41 58 82 115 163 231 326 461<br />
120 1.6 2.3 3.2 4.6 6.5 9.1 12.9 18.3 26 37 52 73 103 146 206 291 412<br />
150 1.2 1.8 2.5 3.5 5.0 7.0 9.9 14.0 19.8 28 40 56 79 112 159 224 317 448<br />
185 1.5 2.1 2.9 4.2 5.9 8.3 11.7 16.6 23 33 47 66 94 133 187 265 374 529<br />
240 1.8 2.6 3.7 5.2 7.3 10.3 14.6 21 29 41 58 83 117 165 233 330 466 659<br />
300 2.2 3.1 4.4 6.2 8.8 12.4 17.6 25 35 50 70 99 140 198 280 396 561<br />
2x120 2.3 3.2 4.6 6.5 9.1 12.9 18.3 26 37 52 73 103 146 206 292 412 583<br />
2x150 2.5 3.5 5.0 7.0 9.9 14.0 20 28 40 56 79 112 159 224 317 448 634<br />
2x185 2.9 4.2 5.9 8.3 11.7 16.6 23 33 47 66 94 133 187 265 375 530 749<br />
553x120 3.4 4.9 6.9 9.7 13.7 19.4 27 39 55 77 110 155 219 309 438 619<br />
3x150 3.7 5.3 7.5 10.5 14.9 21 30 42 60 84 119 168 238 336 476 672<br />
3x185 4.4 6.2 8.8 12.5 17.6 25 35 50 70 100 141 199 281 398 562<br />
Isc upstream Isc downstream<br />
(in kA) (in kA)<br />
100 93 90 87 82 77 70 62 54 45 37 29 22 17.0 12.6 9.3 6.7 4.9 3.5 2.5 1.8 1.3 0.9<br />
90 84 82 79 75 71 65 58 51 43 35 28 22 16.7 12.5 9.2 6.7 4.8 3.5 2.5 1.8 1.3 0.9<br />
80 75 74 71 68 64 59 54 47 40 34 27 21 16.3 12.2 9.1 6.6 4.8 3.5 2.5 1.8 1.3 0.9<br />
70 66 65 63 61 58 54 49 44 38 32 26 20 15.8 12.0 8.9 6.6 4.8 3.4 2.5 1.8 1.3 0.9<br />
60 57 56 55 53 51 48 44 39 35 29 24 20 15.2 11.6 8.7 6.5 4.7 3.4 2.5 1.8 1.3 0.9<br />
50 48 47 46 45 43 41 38 35 31 27 22 18.3 14.5 11.2 8.5 6.3 4.6 3.4 2.4 1.7 1.2 0.9<br />
40 39 38 38 37 36 34 32 30 27 24 20 16.8 13.5 10.6 8.1 6.1 4.5 3.3 2.4 1.7 1.2 0.9<br />
35 34 34 33 33 32 30 29 27 24 22 18.8 15.8 12.9 10.2 7.9 6.0 4.5 3.3 2.4 1.7 1.2 0.9<br />
30 29 29 29 28 27 27 25 24 22 20 17.3 14.7 12.2 9.8 7.6 5.8 4.4 3.2 2.4 1.7 1.2 0.9<br />
25 25 24 24 24 23 23 22 21 19.1 17.4 15.5 13.4 11.2 9.2 7.3 5.6 4.2 3.2 2.3 1.7 1.2 0.9<br />
20 20 20 19.4 19.2 18.8 18.4 17.8 17.0 16.1 14.9 13.4 11.8 10.1 8.4 6.8 5.3 4.1 3.1 2.3 1.7 1.2 0.9<br />
15 14.8 14.8 14.7 14.5 14.3 14.1 13.7 13.3 12.7 11.9 11.0 9.9 8.7 7.4 6.1 4.9 3.8 2.9 2.2 1.6 1.2 0.9<br />
10 9.9 9.9 9.8 9.8 9.7 9.6 9.4 9.2 8.9 8.5 8.0 7.4 6.7 5.9 5.1 4.2 3.4 2.7 2.0 1.5 1.1 0.8<br />
7 7.0 6.9 6.9 6.9 6.9 6.8 6.7 6.6 6.4 6.2 6.0 5.6 5.2 4.7 4.2 3.6 3.0 2.4 1.9 1.4 1.1 0.8<br />
5 5.0 5.0 5.0 4.9 4.9 4.9 4.9 4.8 4.7 4.6 4.5 4.3 4.0 3.7 3.4 3.0 2.5 2.1 1.7 1.3 1.0 0.8<br />
4 4.0 4.0 4.0 4.0 4.0 3.9 3.9 3.9 3.8 3.7 3.6 3.5 3.3 3.1 2.9 2.6 2.2 1.9 1.6 1.2 1.0 0.7<br />
3 3.0 3.0 3.0 3.0 3.0 3.0 2.9 2.9 2.9 2.9 2.8 2.7 2.6 2.5 2.3 2.1 1.9 1.6 1.4 1.1 0.9 0.7<br />
2 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.9 1.9 1.9 1.8 1.8 1.7 1.6 1.4 1.3 1.1 1.0 0.8 0.6<br />
1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 0.7 0.6 0.6 0.5<br />
Aluminium 230 V / 400 V<br />
c.s.a. <strong>of</strong> phase Length <strong>of</strong> circuit (in metres)<br />
conductors (mm 2 )<br />
2.5 1.4 1.9 2.7 3.8 5.4 7.6 10.8 15.3 22<br />
4 1.1 1.5 2.2 3.1 4.3 6.1 8.6 12.2 17.3 24 35<br />
6 1.6 2.3 3.2 4.6 6.5 9.2 13.0 18.3 26 37 52<br />
10 1.9 2.7 3.8 5.4 7.7 10.8 15.3 22 31 43 61 86<br />
16 2.2 3.1 4.3 6.1 8.7 12.2 17.3 24 35 49 69 98 138<br />
25 1.7 2.4 3.4 4.8 6.8 9.6 13.5 19.1 27 38 54 76 108 153 216<br />
35 1.7 2.4 3.4 4.7 6.7 9.5 13.4 18.9 27 38 54 76 107 151 214 302<br />
47.5 1.6 2.3 3.2 4.6 6.4 9.1 12.9 18.2 26 36 51 73 103 145 205 290 410<br />
70 2.4 3.4 4.7 6.7 9.5 13.4 19.0 27 38 54 76 107 151 214 303 428<br />
95 2.3 3.2 4.6 6.4 9.1 12.9 18.2 26 36 51 73 103 145 205 290 411<br />
120 2.9 4.1 5.8 8.1 11.5 16.3 23 32 46 65 92 130 184 259 367<br />
150 3.1 4.4 6.3 8.8 12.5 17.7 25 35 50 71 100 141 199 282 399<br />
185 2.6 3.7 5.2 7.4 10.4 14.8 21 30 42 59 83 118 167 236 333 471<br />
240 1.2 1.6 2.3 3.3 4.6 6.5 9.2 13.0 18.4 26 37 52 73 104 147 208 294 415<br />
300 1.4 2.0 2.8 3.9 5.5 7.8 11.1 15.6 22 31 44 62 88 125 177 250 353 499<br />
2x120 1.4 2.0 2.9 4.1 5.8 8.1 11.5 16.3 23 33 46 65 92 130 184 260 367 519<br />
2x150 1.6 2.2 3.1 4.4 6.3 8.8 12.5 17.7 25 35 50 71 100 141 200 282 399<br />
2x185 1.9 2.6 3.7 5.2 7.4 10.5 14.8 21 30 42 59 83 118 167 236 334 472<br />
2x240 2.3 3.3 4.6 6.5 9.2 13.0 18.4 26 37 52 74 104 147 208 294 415 587<br />
3x120 2.2 3.1 4.3 6.1 8.6 12.2 17.3 24 34 49 69 97 138 195 275 389 551<br />
3x150 2.3 3.3 4.7 6.6 9.4 13.3 18.8 27 37 53 75 106 150 212 299 423 598<br />
3x185 2.8 3.9 5.5 7.8 11.1 15.7 22 31 44 63 89 125 177 250 354 500 707<br />
3x240 3.5 4.9 6.9 9.8 13.8 19.5 28 39 55 78 110 156 220 312 441 623<br />
Note: for a 3-phase system having 230 V between phases, divide the above lengths by 3<br />
Fig. G39 : Isc at a point downstream, as a function <strong>of</strong> a known upstream fault-current value and the length and c.s.a. <strong>of</strong> the intervening conductors,<br />
in a 230/400 V 3-phase system<br />
4.4 Short-circuit current supplied by a generator or<br />
an inverter: Please refer to <strong>Chapter</strong> N<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G29<br />
© Schneider Electric - all rights reserved
G30<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
If a protective device in a circuit is intended<br />
only to protect against short-circuit faults, it is<br />
essential that it will operate with certainty at the<br />
lowest possible level <strong>of</strong> short-circuit current that<br />
can occur on the circuit<br />
Circuit breaker with<br />
instantaneous magnetic<br />
short-circuit protective relay only<br />
Load breaking contactor<br />
with thermal overload relay<br />
Fig. G41 : Circuit protected by circuit-breaker without thermal<br />
overload relay<br />
S1<br />
Load with<br />
incorporated<br />
overload<br />
protection<br />
Circuit breaker D<br />
S2 < S1<br />
Fig. G42a : Circuit-breaker D provides protection against shortcircuit<br />
faults as far as and including the load<br />
5 Particular cases <strong>of</strong> short-circuit<br />
current<br />
5.1 Calculation <strong>of</strong> minimum levels <strong>of</strong> short-circuit<br />
current<br />
In general, on LV circuits, a single protective device protects against all levels <strong>of</strong><br />
current, from the overload threshold through the maximum rated short-circuit currentbreaking<br />
capability <strong>of</strong> the device.<br />
In certain cases, however, overload protective devices and separate short-circuit<br />
protective devices are used.<br />
Examples <strong>of</strong> such arrangements<br />
Figures G40 to G42 show some common arrangements where overload and<br />
short-circuit protections are achieved by separate devices.<br />
Fig. G40 : Circuit protected by aM fuses<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
aM fuses<br />
(no protection<br />
against overload)<br />
Load breaking contactor<br />
with thermal overload relay<br />
As shown in Figures G40 and G41, the most common circuits using separate<br />
devices control and protect motors.<br />
Figure G42a constitutes a derogation in the basic protection <strong>rules</strong>, and is generally<br />
used on circuits <strong>of</strong> prefabricated bustrunking, lighting rails, etc.<br />
Variable speed drive<br />
Figure G42b shows the functions provided by the variable speed drive, and if<br />
necessary some additional functions provided by devices such as circuit-breaker,<br />
thermal relay, RCD.<br />
Protection to be provided Protection generally provided Additional protection<br />
by the variable speed drive<br />
Cable overload Yes = (1) Not necessary if (1)<br />
Motor overload Yes = (2) Not necessary if (2)<br />
Downstream short-circuit Yes<br />
Variable speed drive overload Yes<br />
Overvoltage Yes<br />
Undervoltage Yes<br />
Loss <strong>of</strong> phase Yes<br />
Upstream short-circuit Circuit-breaker<br />
(short-circuit tripping)<br />
Internal fault Circuit-breaker<br />
(short-circuit and<br />
overload tripping)<br />
Downstream earth fault (self protection) RCD u 300 mA<br />
(indirect contact)<br />
Direct contact fault RCD y 30 mA<br />
Figure G42b : Protection to be provided for variable speeed drive applications
G - Sizing and protection <strong>of</strong> conductors<br />
The protective device must fulfill:<br />
b instantaneous trip setting Im < Isc min for a<br />
circuit-breaker<br />
b fusion current Ia < Isc min for a fuse<br />
5 Particular cases <strong>of</strong> short-circuit<br />
current<br />
Conditions to be fulfilled<br />
The protective device must therefore satisfy the two following conditions:<br />
b Its fault-current breaking rating must be greater than Isc, the 3-phase short-circuit<br />
current at its point <strong>of</strong> <strong>installation</strong><br />
b Elimination <strong>of</strong> the minimum short-circuit current possible in the circuit, in a time tc<br />
compatible with the thermal constraints <strong>of</strong> the circuit conductors, where:<br />
2 2<br />
K S (valid for tc < 5 seconds)<br />
tc i<br />
I 2<br />
sc min<br />
Comparison <strong>of</strong> the tripping or fusing performance curve <strong>of</strong> protective devices, with<br />
the limit curves <strong>of</strong> thermal constraint for a conductor shows that this condition is<br />
satisfied if:<br />
b Isc (min) > Im (instantaneous or short timedelay circuit-breaker trip setting current<br />
level), (see Fig. G45)<br />
b Isc (min) > Ia for protection by fuses. The value <strong>of</strong> the current Ia corresponds to<br />
the crossing point <strong>of</strong> the fuse curve and the cable thermal withstand curve<br />
(see Fig. G44 and G45)<br />
Fig. G45 : Protection by circuit-breaker<br />
Fig. G46 : Protection by aM-type fuses<br />
Fig. G47 : Protection by gl-type fuses<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
t<br />
t<br />
t<br />
Im<br />
Ia<br />
Ia<br />
t = k2 S 2<br />
I 2<br />
t = k2 S 2<br />
I 2<br />
t = k2 S 2<br />
I 2<br />
I<br />
I<br />
I<br />
G31<br />
© Schneider Electric - all rights reserved
G32<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
In practice this means that the length <strong>of</strong> circuit<br />
downstream <strong>of</strong> the protective device must not<br />
exceed a calculated maximum length:<br />
0.8 U Sph<br />
Lmax<br />
=<br />
2ρI m<br />
(1) For larger c.s.a.’s, the resistance calculated for the<br />
conductors must be increased to account for the non-uniform<br />
current density in the conductor (due to “skin” and “proximity”<br />
effects)<br />
Suitable values are as follows:<br />
150 mm 2 : R + 15%<br />
185 mm 2 : R + 20%<br />
240 mm 2 : R + 25%<br />
300 mm 2 : R + 30%<br />
(2) Or for aluminium according to conductor material<br />
(3) The high value for resistivity is due to the elevated<br />
temperature <strong>of</strong> the conductor when passing short-circuit<br />
current<br />
5 Particular cases <strong>of</strong> short-circuit<br />
current<br />
Practical method <strong>of</strong> calculating Lmax<br />
The limiting effect <strong>of</strong> the impedance <strong>of</strong> long circuit conductors on the value <strong>of</strong><br />
short-circuit currents must be checked and the length <strong>of</strong> a circuit must be restricted<br />
accordingly.<br />
The method <strong>of</strong> calculating the maximum permitted length has already been<br />
demonstrated in TN- and IT- earthed schemes for single and double earth faults,<br />
respectively (see <strong>Chapter</strong> F Sub-clauses 6.2 and 7.2). Two cases are considered<br />
below:<br />
1 - Calculation <strong>of</strong> L max for a 3-phase 3-wire circuit<br />
The minimum short-circuit current will occur when two phase wires are shortcircuited<br />
at the remote end <strong>of</strong> the circuit (see Fig. G46).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
P<br />
0.8 U<br />
Fig G46 : Definition <strong>of</strong> L for a 3-phase 3-wire circuit<br />
L<br />
Load<br />
Using the “conventional method”, the voltage at the point <strong>of</strong> protection P is assumed<br />
to be 80% <strong>of</strong> the nominal voltage during a short-circuit fault, so that 0.8 U = Isc Zd,<br />
where:<br />
Zd = impedance <strong>of</strong> the fault loop<br />
Isc = short-circuit current (ph/ph)<br />
U = phase-to-phase nominal voltage<br />
For cables y 120 mm2 , reactance may be neglected, so that<br />
2L<br />
Zd = ρ (1)<br />
Sph<br />
where:<br />
ρ = resistivity <strong>of</strong> copper (2) at the average temperature during a short-circuit,<br />
Sph = c.s.a. <strong>of</strong> a phase conductor in mm 2<br />
L = length in metres<br />
The condition for the cable protection is Im y Isc with Im = magnetic trip current<br />
setting <strong>of</strong> the CB.<br />
This leads to Im y<br />
0.8 U<br />
which gives L y 0.8 U Sph<br />
Zd<br />
Lmax<br />
=<br />
2ρI m<br />
with U = 400 V<br />
ρ = 1.25 x 0.018 = 0.023 Ω.mm 2 /m (3)<br />
Lmax = maximum circuit length in metres<br />
k Sph<br />
L<br />
m<br />
max = I<br />
2 - Calculation <strong>of</strong> Lmax for a 3-phase 4-wire 230/400 V circuit<br />
The minimum Isc will occur when the short-circuit is between a phase conductor and<br />
the neutral.<br />
A calculation similar to that <strong>of</strong> example 1 above is required, but using the following<br />
formulae (for cable y 120 mm2 (1) ).<br />
b Where Sn for the neutral conductor = Sph for the phase conductor<br />
3,333 Sph<br />
Lmax<br />
=<br />
Im<br />
b If Sn for the neutral conductor < Sph, then<br />
L 6,666 Sph 1<br />
max =<br />
where m =<br />
Im<br />
1+ m<br />
Sph<br />
Sn<br />
For larger c.s.a.’s than those listed, reactance values must be combined with those <strong>of</strong><br />
resistance to give an impedance. Reactance may be taken as 0.08 mΩ/m for cables<br />
(at 50 Hz). At 60 Hz the value is 0.096 mΩ/m.
G - Sizing and protection <strong>of</strong> conductors<br />
5 Particular cases <strong>of</strong> short-circuit<br />
current<br />
Tabulated values for Lmax<br />
Figure G47 below gives maximum circuit lengths (Lmax) in metres, for:<br />
b 3-phase 4-wire 400 V circuits (i.e. with neutral) and<br />
b 1-phase 2-wire 230 V circuits<br />
protected by general-purpose circuit-breakers.<br />
In other cases, apply correction factors (given in Figure G53) to the lengths obtained.<br />
The calculations are based on the above methods, and a short-circuit trip level within<br />
± 20% <strong>of</strong> the adjusted value Im.<br />
For the 50 mm 2 c.s.a., calculation are based on a 47.5 mm 2 real c.s.a.<br />
Operating current c.s.a. (nominal cross-sectional-area) <strong>of</strong> conductors (in mm 2 )<br />
level Im <strong>of</strong> the<br />
instantaneous<br />
magnetic tripping<br />
element (in A) 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240<br />
50 100 167 267 400<br />
63 79 133 212 317<br />
80 63 104 167 250 417<br />
100 50 83 133 200 333<br />
125 40 67 107 160 267 427<br />
160 31 52 83 125 208 333<br />
200 25 42 67 100 167 267 417<br />
250 20 33 53 80 133 213 333 467<br />
320 16 26 42 63 104 167 260 365 495<br />
400 13 21 33 50 83 133 208 292 396<br />
500 10 17 27 40 67 107 167 233 317<br />
560 9 15 24 36 60 95 149 208 283 417<br />
630 8 13 21 32 63 85 132 185 251 370<br />
700 7 12 19 29 48 76 119 167 226 333 452<br />
800 6 10 17 25 42 67 104 146 198 292 396<br />
875 6 10 15 23 38 61 95 133 181 267 362 457<br />
1000 5 8 13 20 33 53 83 117 158 233 317 400 435<br />
1120 4 7 12 18 30 48 74 104 141 208 283 357 388 459<br />
1250 4 7 11 16 27 43 67 93 127 187 253 320 348 411<br />
1600 5 8 13 21 33 52 73 99 146 198 250 272 321 400<br />
2000 4 7 10 17 27 42 58 79 117 158 200 217 257 320<br />
2500 5 8 13 21 33 47 63 93 127 160 174 206 256<br />
3200 4 6 10 17 26 36 49 73 99 125 136 161 200<br />
4000 5 8 13 21 29 40 58 79 100 109 128 160<br />
5000 4 7 11 17 23 32 47 63 80 87 103 128<br />
6300 5 8 13 19 25 37 50 63 69 82 102<br />
8000 4 7 10 15 20 29 40 50 54 64 80<br />
10000 5 8 12 16 23 32 40 43 51 64<br />
12500 4 7 9 13 19 25 32 35 41 51<br />
Fig. G47 : Maximum circuit lengths in metres for copper conductors (for aluminium, the lengths must be multiplied by 0.62)<br />
Figures G48 to G50 next page give maximum circuit length (Lmax) in metres for:<br />
b 3-phase 4-wire 400 V circuits (i.e. with neutral) and<br />
b 1-phase 2-wire 230 V circuits<br />
protected in both cases by domestic-type circuit-breakers or with circuit-breakers<br />
having similar tripping/current characteristics.<br />
In other cases, apply correction factors to the lengths indicated. These factors are<br />
given in Figure G51 next page.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G33<br />
© Schneider Electric - all rights reserved
G34<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
Fig. G48 : Maximum length <strong>of</strong> copper-conductor circuits in metres protected by B-type circuit-breakers<br />
Fig. G49 : Maximum length <strong>of</strong> copper-conductor circuits in metres protected by C-type circuit-breakers<br />
Fig. G50 : Maximum length <strong>of</strong> copper-conductor circuits in metres protected by D-type circuit-breakers<br />
Fig. G51 : Correction factor to apply to lengths obtained from Figures G47 to G50<br />
5 Particular cases <strong>of</strong> short-circuit<br />
current<br />
Rated current <strong>of</strong> c.s.a. (nominal cross-sectional-area) <strong>of</strong> conductors (in mm 2 )<br />
circuit-breakers (in A) 1.5 2.5 4 6 10 16 25 35 50<br />
6 200 333 533 800<br />
10 120 200 320 480 800<br />
16 75 125 200 300 500 800<br />
20 60 100 160 240 400 640<br />
25 48 80 128 192 320 512 800<br />
32 37 62 100 150 250 400 625 875<br />
40 30 50 80 120 200 320 500 700<br />
50 24 40 64 96 160 256 400 560 760<br />
63 19 32 51 76 127 203 317 444 603<br />
80 15 25 40 60 100 160 250 350 475<br />
100 12 20 32 48 80 128 200 280 380<br />
125 10 16 26 38 64 102 160 224 304<br />
Rated current <strong>of</strong> c.s.a. (nominal cross-sectional-area) <strong>of</strong> conductors (in mm2)<br />
circuit-breakers (in A) 1.5 2.5 4 6 10 16 25 35 50<br />
6 100 167 267 400 667<br />
10 60 100 160 240 400 640<br />
16 37 62 100 150 250 400 625 875<br />
20 30 50 80 120 200 320 500 700<br />
25 24 40 64 96 160 256 400 560 760<br />
32 18.0 31 50 75 125 200 313 438 594<br />
40 15.0 25 40 60 100 160 250 350 475<br />
50 12.0 20 32 48 80 128 200 280 380<br />
63 9.5 16.0 26 38 64 102 159 222 302<br />
80 7.5 12.5 20 30 50 80 125 175 238<br />
100 6.0 10.0 16.0 24 40 64 100 140 190<br />
125 5.0 8.0 13.0 19.0 32 51 80 112 152<br />
Rated current <strong>of</strong> c.s.a. (nominal cross-sectional-area) <strong>of</strong> conductors (in mm2)<br />
circuit-breakers (in A) 1.5 2.5 4 6 10 16 25 35 50<br />
1 429 714<br />
2 214 357 571 857<br />
3 143 238 381 571 952<br />
4 107 179 286 429 714<br />
6 71 119 190 286 476 762<br />
10 43 71 114 171 286 457 714<br />
16 27 45 71 107 179 286 446 625 848<br />
20 21 36 57 86 143 229 357 500 679<br />
25 17.0 29 46 69 114 183 286 400 543<br />
32 13.0 22 36 54 89 143 223 313 424<br />
40 11.0 18.0 29 43 71 114 179 250 339<br />
50 9.0 14.0 23 34 57 91 143 200 271<br />
63 7.0 11.0 18.0 27 45 73 113 159 215<br />
80 5.0 9.0 14.0 21 36 57 89 125 170<br />
100 4.0 7.0 11.0 17.0 29 46 71 100 136<br />
125 3.0 6.0 9.0 14.0 23 37 57 80 109<br />
Circuit detail<br />
3-phase 3-wire 400 V circuit or 1-phase 2-wire 400 V circuit (no neutral) 1.73<br />
1-phase 2-wire (phase and neutral) 230 V circuit 1<br />
3-phase 4-wire 230/400 V circuit or 2-phase 3-wire 230/400 V circuit Sph / S neutral = 1 1<br />
(i.e with neutral) Sph / S neutral = 2 0.67<br />
Note: IEC 60898 accepts an upper short-circuit-current tripping range <strong>of</strong> 10-50 In for<br />
type D circuit-breakers. European standards, and Figure G50 however, are based on<br />
a range <strong>of</strong> 10-20 In, a range which covers the vast majority <strong>of</strong> domestic and similar<br />
<strong>installation</strong>s.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
In general, verification <strong>of</strong> the thermal-withstand<br />
capability <strong>of</strong> a cable is not necessary, except in<br />
cases where cables <strong>of</strong> small c.s.a. are installed<br />
close to, or feeding directly from, the main<br />
general distribution board<br />
5 Particular cases <strong>of</strong> short-circuit<br />
current<br />
Examples<br />
Example 1<br />
In a 1-phase 2-wire <strong>installation</strong> the protection is provided by a 50 A circuit-breaker<br />
type NS80HMA, the instantaneous short-circuit current trip, is set at 500 A (accuracy<br />
<strong>of</strong> ± 20%), i.e. in the worst case would require 500 x 1,2 = 600 A to trip. The cable<br />
c.s.a. = 10 mm 2 and the conductor material is copper.<br />
In Figure G47, the row Im = 500 A crosses the column c.s.a. = 10 mm 2 at the value<br />
for Lmax <strong>of</strong> 67 m. The circuit-breaker protects the cable against short-circuit faults,<br />
therefore, provided that its length does not exceed 67 metres.<br />
Example 2<br />
In a 3-phase 3-wire 400 V circuit (without neutral), the protection is provided by a<br />
220 A circuit-breaker type NS250N with an instantaneous short-circuit current trip<br />
unit type MA set at 2,000 A (± 20%), i.e. a worst case <strong>of</strong> 2,400 A to be certain <strong>of</strong><br />
tripping. The cable c.s.a. = 120 mm 2 and the conductor material is copper.<br />
In Figure G47 the row Im = 2,000 A crosses the column c.s.a. = 120 mm 2 at the<br />
value for Lmax <strong>of</strong> 200 m. Being a 3-phase 3-wire 400 V circuit (without neutral), a<br />
correction factor from Figure G51 must be applied. This factor is seen to be 1.73.<br />
The circuit-breaker will therefore protect the cable against short-circuit current,<br />
provided that its length does not exceed 200 x 1.73= 346 metres.<br />
5.2 Verification <strong>of</strong> the withstand capabilities <strong>of</strong><br />
cables under short-circuit conditions<br />
Thermal constraints<br />
When the duration <strong>of</strong> short-circuit current is brief (several tenths <strong>of</strong> a second<br />
up to five seconds maximum) all <strong>of</strong> the heat produced is assumed to remain in<br />
the conductor, causing its temperature to rise. The heating process is said to be<br />
adiabatic, an assumption that simplifies the calculation and gives a pessimistic result,<br />
i.e. a higher conductor temperature than that which would actually occur, since in<br />
practice, some heat would leave the conductor and pass into the insulation.<br />
For a period <strong>of</strong> 5 seconds or less, the relationship I 2 t = k 2 S 2 characterizes the<br />
time in seconds during which a conductor <strong>of</strong> c.s.a. S (in mm 2 ) can be allowed to<br />
carry a current I, before its temperature reaches a level which would damage the<br />
surrounding insulation.<br />
The factor k 2 is given in Figure G52 below.<br />
Insulation Conductor copper (Cu) Conductor aluminium (Al)<br />
PVC 13,225 5,776<br />
XLPE 20,449 8,836<br />
Fig. G52 : Value <strong>of</strong> the constant k 2<br />
The method <strong>of</strong> verification consists in checking that the thermal energy I 2 t per<br />
ohm <strong>of</strong> conductor material, allowed to pass by the protecting circuit-breaker (from<br />
manufacturers catalogues) is less than that permitted for the particular conductor (as<br />
given in Figure G53 below).<br />
S (mm 2 ) PVC XLPE<br />
Copper Aluminium Copper Aluminium<br />
1.5 0.0297 0.0130 0.0460 0.0199<br />
2.5 0.0826 0.0361 0.1278 0.0552<br />
4 0.2116 0.0924 0.3272 0.1414<br />
6 0.4761 0.2079 0.7362 0.3181<br />
10 1.3225 0.5776 2.0450 0.8836<br />
16 3.3856 1.4786 5.2350 2.2620<br />
25 8.2656 3.6100 12.7806 5.5225<br />
35 16.2006 7.0756 25.0500 10.8241<br />
50 29.839 13.032 46.133 19.936<br />
Fig. G53 : Maximum allowable thermal stress for cables I 2 t (expressed in ampere 2 x second x 10 6 )<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G35<br />
© Schneider Electric - all rights reserved
G36<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
5 Particular cases <strong>of</strong> short-circuit<br />
current<br />
Example<br />
Is a copper-cored XLPE cable <strong>of</strong> 4 mm 2 c.s.a. adequately protected by a<br />
C60N circuit-breaker?<br />
Figure G53 shows that the I 2 t value for the cable is 0.3272 x 10 6 , while the maximum<br />
“let-through” value by the circuit-breaker, as given in the manufacturer’s catalogue, is<br />
considerably less (< 0.1.10 6 A 2 s).<br />
The cable is therefore adequately protected by the circuit-breaker up to its full rated<br />
breaking capability.<br />
Electrodynamic constraints<br />
For all type <strong>of</strong> circuit (conductors or bus-trunking), it is necessary to take<br />
electrodynamic effects into account.<br />
To withstand the electrodynamic constraints, the conductors must be solidly fixed<br />
and the connection must be strongly tightened.<br />
For bus-trunking, rails, etc. it is also necessary to verify that the electrodynamic<br />
withstand performance is satisfactory when carrying short-circuit currents. The peak<br />
value <strong>of</strong> current, limited by the circuit-breaker or fuse, must be less than the busbar<br />
system rating. Tables <strong>of</strong> coordination ensuring adequate protection <strong>of</strong> their products<br />
are generally published by the manufacturers and provide a major advantage <strong>of</strong> such<br />
systems.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
PE<br />
PE<br />
Correct<br />
Incorrect<br />
Fig. G54 : A poor connection in a series arrangement will leave<br />
all downstream appliances unprotected<br />
PEN<br />
Fig. G55 : Direct connection <strong>of</strong> the PEN conductor to the earth<br />
terminal <strong>of</strong> an appliance<br />
6 Protective earthing conductor<br />
(PE)<br />
6.1 Connection and choice<br />
Protective (PE) conductors provide the bonding connection between all exposed<br />
and extraneous conductive parts <strong>of</strong> an <strong>installation</strong>, to create the main equipotential<br />
bonding system. These conductors conduct fault current due to insulation failure<br />
(between a phase conductor and an exposed conductive part) to the earthed neutral<br />
<strong>of</strong> the source. PE conductors are connected to the main earthing terminal <strong>of</strong> the<br />
<strong>installation</strong>.<br />
The main earthing terminal is connected to the earthing electrode (see <strong>Chapter</strong> E)<br />
by the earthing conductor (grounding electrode conductor in the USA).<br />
PE conductors must be:<br />
b Insulated and coloured yellow and green (stripes)<br />
b Protected against mechanical and chemical damage<br />
In IT and TN-earthed schemes it is strongly recommended that PE conductors<br />
should be installed in close proximity (i.e. in the same conduits, on the same cable<br />
tray, etc.) as the live cables <strong>of</strong> the related circuit. This arrangement ensures the<br />
minimum possible inductive reactance in the earth-fault current carrying circuits.<br />
It should be noted that this arrangement is originally provided by bus-trunking.<br />
Connection<br />
PE conductors must:<br />
b Not include any means <strong>of</strong> breaking the continuity <strong>of</strong> the circuit (such as a switch,<br />
removable links, etc.)<br />
b Connect exposed conductive parts individually to the main PE conductor, i.e. in<br />
parallel, not in series, as shown in Figure G54<br />
b Have an individual terminal on common earthing bars in distribution boards.<br />
TT scheme<br />
The PE conductor need not necessarily be installed in close proximity to the live<br />
conductors <strong>of</strong> the corresponding circuit, since high values <strong>of</strong> earth-fault current are<br />
not needed to operate the RCD-type <strong>of</strong> protection used in TT <strong>installation</strong>s.<br />
IT and TN schemes<br />
The PE or PEN conductor, as previously noted, must be installed as close as<br />
possible to the corresponding live conductors <strong>of</strong> the circuit and no ferro-magnetic<br />
material must be interposed between them. A PEN conductor must always be<br />
connected directly to the earth terminal <strong>of</strong> an appliance, with a looped connection<br />
from the earth terminal to the neutral terminal <strong>of</strong> the appliance (see Fig. G55).<br />
b TN-C scheme (the neutral and PE conductor are one and the same, referred to as<br />
a PEN conductor)<br />
The protective function <strong>of</strong> a PEN conductor has priority, so that all <strong>rules</strong> governing<br />
PE conductors apply strictly to PEN conductors<br />
b TN-C to TN-S transition<br />
The PE conductor for the <strong>installation</strong> is connected to the PEN terminal or bar (see<br />
Fig. G56) generally at the origin <strong>of</strong> the <strong>installation</strong>. Downstream <strong>of</strong> the point <strong>of</strong><br />
separation, no PE conductor can be connected to the neutral conductor.<br />
Fig. G56 : The TN-C-S scheme<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
PEN PE<br />
N<br />
G37<br />
© Schneider Electric - all rights reserved
G38<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
Fig. G57 : Choice <strong>of</strong> protective conductors (PE)<br />
Fig. G58 : Minimum cross section area <strong>of</strong> protective conductors<br />
Types <strong>of</strong> materials<br />
Materials <strong>of</strong> the kinds mentioned below in Figure G57 can be used for PE conductors,<br />
provided that the conditions mentioned in the last column are satisfied.<br />
Type <strong>of</strong> protective earthing conductor (PE) IT scheme TN scheme TT scheme Conditions to be respected<br />
Supplementary In the same cable Strongly Strongly recommended Correct The PE conductor must<br />
conductor as the phases, or in recommended be insulated to the same<br />
the same cable run level as the phases<br />
Independent <strong>of</strong> the Possible (1) Possible (1) (2) Correct b The PE conductor may<br />
phase conductors be bare or insulated (2)<br />
Metallic housing <strong>of</strong> bus-trunking or <strong>of</strong> other Possible (3) PE possible (3) Correct b The <strong>electrical</strong> continuity<br />
prefabricated prewired ducting (5) PEN possible (8) must be assured by protection<br />
External sheath <strong>of</strong> extruded, mineral- insulated Possible (3) PE possible (3) Possible against deterioration by<br />
conductors (e.g. «pyrotenax» type systems) PEN not recommended (2)(3) Certain extraneous conductive elements<br />
mechanical, chemical and<br />
(6) Possible (4) PE possible (4) Possible<br />
electrochemical hazards<br />
such as: PEN forbidden b Their conductance<br />
b Steel building structures<br />
b Machine frames<br />
b Water pipes<br />
must be adequate<br />
(7)<br />
Metallic cable ways, such as, conduits (9) , Possible (4) PE possible (4) Possible<br />
ducts, trunking, trays, ladders, and so on… PEN not recommended (2)(4)<br />
Forbidden for use as PE conductors, are: metal conduits (9) , gas pipes, hot-water pipes, cable-armouring tapes (9) or wires (9)<br />
(1) In TN and IT schemes, fault clearance is generally achieved by overcurrent devices (fuses or circuit-breakers) so that the impedance<br />
<strong>of</strong> the fault-current loop must be sufficiently low to assure positive protective device operation. The surest means <strong>of</strong> achieving a low loop<br />
impedance is to use a supplementary core in the same cable as the circuit conductors (or taking the same route as the circuit conductors).<br />
This solution minimizes the inductive reactance and therefore the impedance <strong>of</strong> the loop.<br />
(2) The PEN conductor is a neutral conductor that is also used as a protective earth conductor. This means that a current may be flowing<br />
through it at any time (in the absence <strong>of</strong> an earth fault). For this reason an insulated conductor is recommended for PEN operation.<br />
(3) The manufacturer provides the necessary values <strong>of</strong> R and X components <strong>of</strong> the impedances (phase/PE, phase/PEN) to include in the<br />
calculation <strong>of</strong> the earth-fault loop impedance.<br />
(4) Possible, but not recomended, since the impedance <strong>of</strong> the earth-fault loop cannot be known at the <strong>design</strong> stage. Measurements on the<br />
completed <strong>installation</strong> are the only practical means <strong>of</strong> assuring adequate protection for persons.<br />
(5) It must allow the connection <strong>of</strong> other PE conductors. Note: these elements must carry an indivual green/yellow striped visual indication,<br />
15 to 100 mm long (or the letters PE at less than 15 cm from each extremity).<br />
(6) These elements must be demountable only if other means have been provided to ensure uninterrupted continuity <strong>of</strong> protection.<br />
(7) With the agreement <strong>of</strong> the appropriate water authorities.<br />
(8) In the prefabricated pre-wired trunking and similar elements, the metallic housing may be used as a PEN conductor, in parallel with the<br />
corresponding bar, or other PE conductor in the housing.<br />
(9) Forbidden in some countries only. Universally allowed to be used for supplementary equipotential conductors.<br />
6.2 Conductor sizing<br />
Figure G58 below is based on IEC 60364-5-54. This table provides two methods <strong>of</strong><br />
determining the appropriate c.s.a. for both PE or PEN conductors.<br />
c.s.a. <strong>of</strong> phase Minimum c.s.a. <strong>of</strong> Minimum c.s.a. <strong>of</strong><br />
conductors Sph (mm2 ) PE conductor (mm2 ) PEN conductor (mm2 )<br />
Cu Al<br />
Simplified Sph y 16 S (2)<br />
ph Sph<br />
(3)<br />
Sph<br />
(3)<br />
method (1) 16 < Sph y 25 16 16<br />
25 < Sph y 35 25<br />
35 < Sph y 50 Sph /2 Sph /2<br />
Sph > 50 S Sph /2<br />
Adiabatic method Any size<br />
6 Protective earthing conductor<br />
(PE)<br />
SPE/PEN<br />
=<br />
(1) Data valid if the prospective conductor is <strong>of</strong> the same material as the line conductor. Otherwise, a correction factor must be applied.<br />
(2) When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected:<br />
b 2.5 mm 2 if the PE is mechanically protected<br />
b 4 mm 2 if the PE is not mechanically protected<br />
(3) For mechanical reasons, a PEN conductor, shall have a cross-sectional area not less than 10 mm 2 in copper or 16 mm 2 in aluminium.<br />
(4) Refer to table G53 for the application <strong>of</strong> this formula.<br />
I ⋅t<br />
k<br />
2<br />
(3) (4)<br />
(3) (4)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
(1) Grounding electrode conductor<br />
6 Protective earthing conductor<br />
(PE)<br />
The two methods are:<br />
b Adiabatic (which corresponds with that described in IEC 60724)<br />
This method, while being economical and assuring protection <strong>of</strong> the conductor<br />
against overheating, leads to small c.s.a.’s compared to those <strong>of</strong> the corresponding<br />
circuit phase conductors. The result is sometimes incompatible with the necessity<br />
in IT and TN schemes to minimize the impedance <strong>of</strong> the circuit earth-fault loop, to<br />
ensure positive operation by instantaneous overcurrent tripping devices. This method<br />
is used in practice, therefore, for TT <strong>installation</strong>s, and for dimensioning an earthing<br />
conductor (1) .<br />
b Simplified<br />
This method is based on PE conductor sizes being related to those <strong>of</strong> the<br />
corresponding circuit phase conductors, assuming that the same conductor material<br />
is used in each case.<br />
Thus, in Figure G58 for:<br />
Sph y 16 mm 2 SPE = Sph<br />
16 < Sph y 35 mm2 SPE = 16 mm2 Sph > 35 mm2 Sph<br />
Sph > 35 mm2 SPE<br />
=<br />
2<br />
: when, in a TT scheme, the <strong>installation</strong> earth electrode is beyond the zone <strong>of</strong><br />
Note: when, in a TT scheme, the <strong>installation</strong> earth electrode is beyond the zone <strong>of</strong><br />
influence <strong>of</strong> the source earthing electrode, the c.s.a. <strong>of</strong> the PE conductor can be<br />
limited to 25 mm2 (for copper) or 35 mm2 (for aluminium).<br />
The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger<br />
than 10 mm 2 (copper) or 16 mm 2 (aluminium).<br />
Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor<br />
functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that<br />
necessary for the neutral, as discussed in Subclause 7.1 <strong>of</strong> this <strong>Chapter</strong>.<br />
This c.s.a. cannot be less than that <strong>of</strong> the phase conductors unless:<br />
b The kVA rating <strong>of</strong> single-phase loads is less than 10% <strong>of</strong> the total kVA load, and<br />
b Imax likely to pass through the neutral in normal circumstances, is less than the<br />
current permitted for the selected cable size.<br />
Furthermore, protection <strong>of</strong> the neutral conductor must be assured by the protective<br />
devices provided for phase-conductor protection (described in Sub-clause 7.2 <strong>of</strong> this<br />
<strong>Chapter</strong>).<br />
Values <strong>of</strong> factor k to be used in the formulae<br />
These values are identical in several national standards, and the temperature rise<br />
ranges, together with factor k values and the upper temperature limits for the different<br />
classes <strong>of</strong> insulation, correspond with those published in IEC 60724 (1984).<br />
The data presented in Figure G59 are those most commonly needed for LV <strong>installation</strong><br />
<strong>design</strong>.<br />
k values Nature <strong>of</strong> insulation<br />
Polyvinylchloride (PVC) Cross-linked-polyethylene<br />
(XLPE)<br />
Ethylene-propylene-rubber<br />
(EPR)<br />
Final temperature (°C) 160 250<br />
Initial temperature (°C) 30 30<br />
Insulated conductors Copper 143 176<br />
not incoporated in<br />
cables or bare<br />
conductors in contact<br />
with cable jackets<br />
Aluminium<br />
Steel<br />
95<br />
52<br />
116<br />
64<br />
Conductors <strong>of</strong> a Copper 115 143<br />
multi-core-cable Aluminium 76 94<br />
Fig. G59 : k factor values for LV PE conductors, commonly used in national standards and<br />
complying with IEC 60724<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G39<br />
© Schneider Electric - all rights reserved
G40<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
These conductors must be sized according to<br />
national practices<br />
6 Protective earthing conductor<br />
(PE)<br />
6.3 Protective conductor between MV/LV transformer<br />
and the main general distribution board (MGDB)<br />
All phase and neutral conductors upstream <strong>of</strong> the main incoming circuit-breaker<br />
controlling and protecting the MGDB are protected by devices at the MV side <strong>of</strong> the<br />
transformer. The conductors in question, together with the PE conductor, must be<br />
dimensioned accordingly. Dimensioning <strong>of</strong> the phase and neutral conductors from<br />
the transformer is exemplified in Sub-clause 7.5 <strong>of</strong> this chapter (for circuit C1 <strong>of</strong> the<br />
system illustrated in Fig. G65).<br />
Recommended conductor sizes for bare and insulated PE conductors from the<br />
transformer neutral point, shown in Figure G60, are indicated below in Figure G61.<br />
The kVA rating to consider is the sum <strong>of</strong> all (if more than one) transformers<br />
connected to the MGDB.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
PE<br />
Main earth bar<br />
for the LV <strong>installation</strong><br />
Fig. G60 : PE conductor to the main earth bar in the MGDB<br />
MGDB<br />
The table indicates the c.s.a. <strong>of</strong> the conductors in mm 2 according to:<br />
b The nominal rating <strong>of</strong> the MV/LV transformer(s) in kVA<br />
b The fault-current clearance time by the MV protective devices, in seconds<br />
b The kinds <strong>of</strong> insulation and conductor materials<br />
If the MV protection is by fuses, then use the 0.2 seconds columns.<br />
In IT schemes, if an overvoltage protection device is installed (between the<br />
transformer neutral point and earth) the conductors for connection <strong>of</strong> the device<br />
should also be dimensioned in the same way as that described above for<br />
PE conductors.<br />
Transformer Conductor Bare PVC-insulated XLPE-insulated<br />
rating in kVA material conductors conductors conductors<br />
(230/400 V Copper t(s) 0.2 0.5 - 0.2 0.5 - 0.2 0.5 -<br />
output) Aluminium t(s) - 0.2 0.5 - 0.2 0.5 - 0.2 0.5<br />
y100 c.s.a. <strong>of</strong> PE 25 25 25 25 25 25 25 25 25<br />
160 conductors 25 25 35 25 25 50 25 25 35<br />
200 SPE (mm 2 ) 25 35 50 25 35 50 25 25 50<br />
250 25 35 70 35 50 70 25 35 50<br />
315 35 50 70 35 50 95 35 50 70<br />
400 50 70 95 50 70 95 35 50 95<br />
500 50 70 120 70 95 120 50 70 95<br />
630 70 95 150 70 95 150 70 95 120<br />
800 70 120 150 95 120 185 70 95 150<br />
1,000 95 120 185 95 120 185 70 120 150<br />
1,250 95 150 185 120 150 240 95 120 185<br />
Fig. G61 : Recommended c.s.a. <strong>of</strong> PE conductor between the MV/LV transformer and the MGDB,<br />
as a function <strong>of</strong> transformer ratings and fault-clearance times.
G - Sizing and protection <strong>of</strong> conductors<br />
6 Protective earthing conductor<br />
(PE)<br />
6.4 Equipotential conductor<br />
The main equipotential conductor<br />
This conductor must, in general, have a c.s.a. at least equal to half <strong>of</strong> that <strong>of</strong> the<br />
largest PE conductor, but in no case need exceed 25 mm 2 (copper) or 35 mm 2<br />
(aluminium) while its minimum c.s.a. is 6 mm 2 (copper) or 10 mm 2 (aluminium).<br />
Supplementary equipotential conductor<br />
This conductor allows an exposed conductive part which is remote from the nearest<br />
main equipotential conductor (PE conductor) to be connected to a local protective<br />
conductor. Its c.s.a. must be at least half <strong>of</strong> that <strong>of</strong> the protective conductor to which<br />
it is connected.<br />
If it connects two exposed conductive parts (M1 and M2 in Figure G62) its c.s.a.<br />
must be at least equal to that <strong>of</strong> the smaller <strong>of</strong> the two PE conductors (for M1 and<br />
M2). Equipotential conductors which are not incorporated in a cable, should be<br />
protected mechanically by conduits, ducting, etc. wherever possible.<br />
Other important uses for supplementary equipotential conductors concern the<br />
reduction <strong>of</strong> the earth-fault loop impedance, particulary for indirect-contact protection<br />
schemes in TN- or IT-earthed <strong>installation</strong>s, and in special locations with increased<br />
<strong>electrical</strong> risk (refer to IEC 60364-4-41).<br />
Between two exposed conductive parts<br />
if SPE1 y SPE2<br />
then SLS = SPE1<br />
M1<br />
SPE1 SPE2<br />
SPE1<br />
M2<br />
Fig. G62 : Supplementary equipotential conductors<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Between an exposed conductive part<br />
and a metallic structure<br />
SLS =<br />
SPE<br />
SLS SLS<br />
2<br />
M1<br />
Metal structures<br />
(conduits, girders…)<br />
G41<br />
© Schneider Electric - all rights reserved
G42<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
(1) Harmonics <strong>of</strong> order 3 and multiple <strong>of</strong> 3<br />
I 1 H1<br />
Fig. G63a : Triplen harmonics are in phase and cumulate in the Neutral<br />
7 The neutral conductor<br />
The c.s.a. and the protection <strong>of</strong> the neutral conductor, apart from its current-carrying<br />
requirement, depend on several factors, namely:<br />
b The type <strong>of</strong> earthing system, TT, TN, etc.<br />
b The harmonic currents<br />
b The method <strong>of</strong> protection against indirect contact hazards according to the<br />
methods described below<br />
The color <strong>of</strong> the neutral conductor is statutorily blue. PEN conductor, when insulated,<br />
shall be marked by one <strong>of</strong> the following methods :<br />
b Green-and-yellow throughout its length with, in addition, light blue markings at the<br />
terminations, or<br />
b Light blue throughout its length with, in addition, green-and-yellow markings at the<br />
terminations<br />
7.1 Sizing the neutral conductor<br />
Influence <strong>of</strong> the type <strong>of</strong> earthing system<br />
TT and TN-S schemes<br />
b Single-phase circuits or those <strong>of</strong> c.s.a. y 16 mm 2 (copper) 25 mm 2 (aluminium): the<br />
c.s.a. <strong>of</strong> the neutral conductor must be equal to that <strong>of</strong> the phases<br />
b Three-phase circuits <strong>of</strong> c.s.a. > 16 mm 2 copper or 25 mm 2 aluminium: the c.s.a. <strong>of</strong><br />
the neutral may be chosen to be:<br />
v Equal to that <strong>of</strong> the phase conductors, or<br />
v Smaller, on condition that:<br />
- The current likely to flow through the neutral in normal conditions is less than the<br />
permitted value Iz. The influence <strong>of</strong> triplen (1) harmonics must be given particular<br />
consideration or<br />
- The neutral conductor is protected against short-circuit, in accordance with the<br />
following Sub-clause G-7.2<br />
- The size <strong>of</strong> the neutral conductor is at least equal to 16 mm 2 in copper or 25 mm 2 in<br />
aluminium<br />
TN-C scheme<br />
The same conditions apply in theory as those mentioned above, but in practice,<br />
the neutral conductor must not be open-circuited under any circumstances since<br />
it constitutes a PE as well as a neutral conductor (see Figure G58 “c.s.a. <strong>of</strong> PEN<br />
conductor” column).<br />
IT scheme<br />
In general, it is not recommended to distribute the neutral conductor, i.e. a 3-phase<br />
3-wire scheme is preferred. When a 3-phase 4-wire <strong>installation</strong> is necessary,<br />
however, the conditions described above for TT and TN-S schemes are applicable.<br />
Influence <strong>of</strong> harmonic currents<br />
Effects <strong>of</strong> triplen harmonics<br />
Harmonics are generated by the non-linear loads <strong>of</strong> the <strong>installation</strong> (computers,<br />
fluorescent lighting, rectifiers, power electronic choppers) and can produce high<br />
currents in the Neutral. In particular triplen harmonics <strong>of</strong> the three Phases have a<br />
tendency to cumulate in the Neutral as:<br />
b Fundamental currents are out-<strong>of</strong>-phase by 2π/3 so that their sum is zero<br />
b On the other hand, triplen harmonics <strong>of</strong> the three Phases are always positioned in<br />
the same manner with respect to their own fundamental, and are in phase with each<br />
other (see Fig. G63a).<br />
+<br />
I 1 H3<br />
I 2 H1 + I 2 H3<br />
I 3 H1<br />
I N =<br />
3<br />
1<br />
I k H1<br />
+<br />
+<br />
I 3 H3<br />
I k H3<br />
0 + 3<br />
IH3 Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
3<br />
1
G - Sizing and protection <strong>of</strong> conductors<br />
7 The neutral conductor<br />
Figure G63b shows the load factor <strong>of</strong> the neutral conductor as a function <strong>of</strong> the<br />
percentage <strong>of</strong> 3 rd harmonic.<br />
In practice, this maximum load factor cannot exceed 3.<br />
2.0<br />
1.8<br />
1.6<br />
1.4<br />
1.2<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
I Neutral<br />
I Phase<br />
Fig. G63b : Load factor <strong>of</strong> the neutral conductor vs the percentage <strong>of</strong> 3 rd harmonic<br />
Reduction factors for harmonic currents in four-core and five-core cables with<br />
four cores carrying current<br />
The basic calculation <strong>of</strong> a cable concerns only cables with three loaded conductors<br />
i.e there is no current in the neutral conductor. Because <strong>of</strong> the third harmonic current,<br />
there is a current in the neutral. As a result, this neutral current creates an hot<br />
environment for the 3 phase conductors and for this reason, a reduction factor for<br />
phase conductors is necessary (see Fig. G63).<br />
Reduction factors, applied to the current-carrying capacity <strong>of</strong> a cable with three<br />
loaded conductors, give the current-carrying capacity <strong>of</strong> a cable with four loaded<br />
conductors, where the current in the fourth conductor is due to harmonics. The<br />
reduction factors also take the heating effect <strong>of</strong> the harmonic current in the phase<br />
conductors into account.<br />
b Where the neutral current is expected to be higher than the phase current, then the<br />
cable size should be selected on the basis <strong>of</strong> the neutral current<br />
b Where the cable size selection is based on a neutral current which is not<br />
significantly higher than the phase current, it is necessary to reduce the tabulated<br />
current carrying capacity for three loaded conductors<br />
b If the neutral current is more than 135% <strong>of</strong> the phase current and the cable size is<br />
selected on the basis <strong>of</strong> the neutral current then the three phase conductors will not<br />
be fully loaded. The reduction in heat generated by the phase conductors <strong>of</strong>fsets the<br />
heat generated by the neutral conductor to the extent that it is not necessary to apply<br />
any reduction factor to the current carrying capacity for three loaded conductors.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
0<br />
0 20 40 60 80 100<br />
i3 (%)<br />
Third harmonic content Reduction factor<br />
<strong>of</strong> phase current Size selection is based on Size selection is based on<br />
(%) phase current neutral current<br />
0 - 15 1.0 -<br />
15 - 33 0.86 -<br />
33 - 45 - 0.86<br />
> 45 - 1.0<br />
Fig. G63 : Reduction factors for harmonic currents in four-core and five-core cables<br />
(according to IEC 60364-5-52)<br />
G43<br />
© Schneider Electric - all rights reserved
G44<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
7 The neutral conductor<br />
Examples<br />
Consider a three-phase circuit with a <strong>design</strong> load <strong>of</strong> 37 A to be installed using fourcore<br />
PVC insulated cable clipped to a wall, <strong>installation</strong> method C. From Figure G24,<br />
a 6 mm 2 cable with copper conductors has a current-carrying capacity <strong>of</strong> 40 A and<br />
hence is suitable if harmonics are not present in the circuit.<br />
b If 20 % third harmonic is present, then a reduction factor <strong>of</strong> 0,86 is applied and the<br />
<strong>design</strong> load becomes: 37/0.86 = 43 A.<br />
For this load a 10 mm 2 cable is necessary.<br />
b If 40 % third harmonic is present, the cable size selection is based on the neutral<br />
current which is: 37 x 0,4 x 3 = 44,4 A and a reduction factor <strong>of</strong> 0,86 is applied,<br />
leading to a <strong>design</strong> load <strong>of</strong>: 44.4/0.86 = 51.6 A.<br />
For this load a 10 mm 2 cable is suitable.<br />
b If 50 % third harmonic is present, the cable size is again selected on the basis <strong>of</strong><br />
the neutral current, which is: 37 x 0,5 x 3 = 55,5 A .In this case the rating factor is<br />
1 and a 16 mm 2 cable is required.<br />
7.2 Protection <strong>of</strong> the neutral conductor<br />
(see Fig. G64 next page)<br />
Protection against overload<br />
If the neutral conductor is correctly sized (including harmonics), no specific<br />
protection <strong>of</strong> the neutral conductor is required because it is protected by the phase<br />
protection.<br />
However, in practice, if the c.s.a. <strong>of</strong> the neutral conductor is lower than the phase<br />
c.s.a, a neutral overload protection must be installed.<br />
Protection against short-circuit<br />
If the c.s.a. <strong>of</strong> the neutral conductor is lower than the c.s.a. <strong>of</strong> the phase conductor,<br />
the neutral conductor must be protected against short-circuit.<br />
If the c.s.a. <strong>of</strong> the neutral conductor is equal or greater than the c.s.a. <strong>of</strong> the phase<br />
conductor, no specific protection <strong>of</strong> the neutral conductor is required because it is<br />
protected by the phase protection.<br />
7.3 Breaking <strong>of</strong> the neutral conductor<br />
(see Fig. G64 next page)<br />
The need to break or not the neutral conductor is related to the protection against<br />
indirect contact.<br />
In TN-C scheme<br />
The neutral conductor must not be open-circuited under any circumstances since it<br />
constitutes a PE as well as a neutral conductor.<br />
In TT, TN-S and IT schemes<br />
In the event <strong>of</strong> a fault, the circuit-breaker will open all poles, including the neutral<br />
pole, i.e. the circuit-breaker is omnipolar.<br />
The action can only be achieved with fuses in an indirect way, in which the operation<br />
<strong>of</strong> one or more fuses triggers a mechanical trip-out <strong>of</strong> all poles <strong>of</strong> an associated<br />
series-connected load-break switch.<br />
7.4 Isolation <strong>of</strong> the neutral conductor<br />
(see Fig. G64 next page)<br />
It is considered to be the good practice that every circuit be provided with the means<br />
for its isolation.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
Single-phase<br />
(Phase-Neutral)<br />
Single-phase<br />
(Phase-Phase)<br />
Three-phase<br />
four wires<br />
Sn u Sph<br />
Three-phase<br />
four wires<br />
Sn < Sph<br />
Fig. G64 : The various situations in which the neutral conductor may appear<br />
TT TN-C TN-S IT<br />
N N N N<br />
or or<br />
N<br />
7 The neutral conductor<br />
(A) (A)<br />
or or<br />
N N N<br />
N<br />
or<br />
N N N<br />
or<br />
(A) Authorized for TT or TN-S systems if a RCD is installed at the origin <strong>of</strong> the circuit or upstream <strong>of</strong> it, and if no artificial<br />
neutral is distributed downstream <strong>of</strong> its location<br />
(B) The neutral overcurrent protection is not necessary:<br />
b If the neutral conductor is protected against short-circuits by a device placed upstream, or,<br />
b If the circuit is protected by a RCD which sensitivity is less than 15% <strong>of</strong> the neutral admissible current.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
N<br />
N<br />
N<br />
(B)<br />
(B)<br />
(B)<br />
G45<br />
© Schneider Electric - all rights reserved
G46<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
Q9<br />
C9<br />
L9<br />
B8<br />
x1<br />
Fig. G65 : Example <strong>of</strong> single-line diagram<br />
Q6<br />
C6<br />
T6<br />
Q7<br />
C7<br />
B2<br />
Circuit 9<br />
T1<br />
C1<br />
Q1<br />
Circuit 6<br />
P = 400 kVA<br />
U = 400 V<br />
Circuit 7<br />
ku = 1.0<br />
ib = 250.00 A<br />
P = 147.22 kW<br />
8 Worked example <strong>of</strong> cable<br />
calculation<br />
Worked example <strong>of</strong> cable calculation (see Fig. G65)<br />
The <strong>installation</strong> is supplied through a 1,000 kVA transformer. The process requires<br />
a high degree <strong>of</strong> supply continuity and this is provided by the <strong>installation</strong> <strong>of</strong> a 500 kVA<br />
400 V standby generator and the adoption <strong>of</strong> a 3-phase 3-wire IT system at the main<br />
general distribution board. The remainder <strong>of</strong> the <strong>installation</strong> is isolated by a 400 kVA<br />
400/400 V transformer. The downstream network is a TT-earthed 3-phase 4-wire<br />
system. Following the single-line diagram shown in Figure G65 below, a reproduction<br />
<strong>of</strong> the results <strong>of</strong> a computer study for the circuit C1, the circuit-breaker Q1, the circuit<br />
C6 and the circuit-breaker Q6. These studies were carried out with ECODIAL 3.3<br />
s<strong>of</strong>tware (a Merlin Gerin product).<br />
This is followed by the same calculations carried out by the method described in this<br />
guide.<br />
1000 kVA 400 V 50 Hz<br />
Circuit 1<br />
Q10<br />
C10<br />
L10<br />
Q3<br />
B4<br />
x1<br />
Switchboard 2<br />
Ks = 1.00<br />
ib = 826.8 A<br />
Q12<br />
C12<br />
L12<br />
Circuit 10<br />
ku = 1.0<br />
ib = 160.00 A<br />
P = 94.22 kW<br />
x1<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G5<br />
C5<br />
Q5<br />
Circuit 12<br />
G<br />
ku = 1.0<br />
ib = 250.00 A<br />
P = 147.22 kW<br />
Q11<br />
C11<br />
L11<br />
x1<br />
P = 500 kVA<br />
U = 400 V<br />
Circuit 5<br />
Switchboard 8<br />
Ks = 1.00<br />
ib = 490.0 A<br />
Circuit 11<br />
Switchboard 4<br />
Ks = 1.00<br />
ib = 250.0 A<br />
ku = 1.0<br />
ib = 80.00 A<br />
P = 47.11 kW
G - Sizing and protection <strong>of</strong> conductors<br />
<strong>General</strong> network characteristics<br />
Earthing system IT<br />
Neutral distributed No<br />
Voltage (V) 400<br />
Frequency (Hz) 50<br />
Transformer T1<br />
Number <strong>of</strong> transformers 1<br />
Upstream fault level (MVA) 500<br />
Rating (kVA) 1,000<br />
Short-circuit impedance voltage (%) 6<br />
Resistance <strong>of</strong> MV network (mΩ) 0.0351<br />
Reactance <strong>of</strong> MV network (mΩ) 0.351<br />
Transformer resistance RT (mΩ) 2.293<br />
Transformer reactance XT (mΩ) 10.333<br />
3-phase short-circuit current Ik3 (kA) 23.3<br />
Cable C1<br />
Maximum load current (A) 1,374<br />
Type <strong>of</strong> insulation PVC<br />
Conductor material Copper<br />
Ambient temperature (°C) 30<br />
Single-core or multi-core cable Single<br />
Installation method F<br />
Number <strong>of</strong> circuits in close proximity (table G21b) 1<br />
Other coefficient 1<br />
Selected cross-sectional area (mm 2 ) 6 x 95<br />
Protective conductor 1 x 120<br />
Length (m) 5<br />
Voltage drop ΔU (%) .122<br />
Voltage drop ΔU total (%) .122<br />
3-phase short-circuit current Ik3 (kA) 23<br />
1-phase-to-earth fault current Id (kA) 17<br />
Circuit-breaker Q1<br />
3-ph short-circuit current Ik3 upstream<br />
<strong>of</strong> the circuit-breaker (kA) 23<br />
Maximum load current (A) 1,374<br />
Number <strong>of</strong> poles and protected poles 3P3D<br />
Circuit-breaker NT 16<br />
Type H 1 – 42 kA<br />
Tripping unit type Micrologic 5 A<br />
Rated current (A) 1,600<br />
Fig. G66 : Partial results <strong>of</strong> calculation carried out with Ecodial s<strong>of</strong>tware (Merlin Gerin)<br />
8 Worked example <strong>of</strong> cable<br />
calculation<br />
Calculation using s<strong>of</strong>tware Ecodial 3.3<br />
The same calculation using the simplified method<br />
recommended in this guide<br />
Dimensioning circuit C1<br />
The MV/LV 1,000 kVA transformer has a rated no-load voltage <strong>of</strong> 420 V. Circuit C1<br />
must be suitable for a current <strong>of</strong><br />
3<br />
1,000 x 10<br />
IB = = 1,374 A per phase<br />
3 x 420<br />
Six single-core PVC-insulated copper cables in parallel will be used for each phase.<br />
These cables will be laid on cable trays according to method F. The “k” correction<br />
factors are as follows:<br />
k 1 = 1 (see table G12, temperature = 30 °C)<br />
k 4 = 0.87 (see table G17, touching cables, 1 tray, u 3 circuits)<br />
Other correction factors are not relevant in this example.<br />
The corrected load current is:<br />
IB<br />
1,374<br />
I' B = = = 1,579 A<br />
k1⋅k4 0. 87<br />
Each conductor will therefore carry 263 A. Figure G21a G23 indicates that the the c.s.a. is is<br />
95 mm2 .<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Busbars B2<br />
Maximum load current (A) 1,374<br />
Type Standard on<br />
edge<br />
Ambient temperature (°C) 30<br />
Dimensions (m and mm) 1 m<br />
2x5 mm x 63 mm<br />
Material Copper<br />
3-ph short-circuit current Ik3 (kA) 23<br />
3-ph peak value <strong>of</strong> short-circuit current Ik (kA) 48<br />
Resistance <strong>of</strong> busbar R (mΩ) 2.52<br />
Reactance <strong>of</strong> busbar X (mΩ) 10.8<br />
Circuit-breaker Q6<br />
3-ph short-circuit current upstream<br />
<strong>of</strong> the circuit-breaker Ik3 (kA) 23<br />
Maximum load current (A) 560<br />
Number <strong>of</strong> poles and protected poles 3P3D<br />
Circuit-breaker NS800<br />
Type N – 50 kA<br />
Tripping unit type Micrologic 2.0<br />
Rated current (A) 800<br />
Limit <strong>of</strong> discrimination (kA) Total<br />
Cable C6<br />
Maximum load current (A) 560<br />
Type <strong>of</strong> insulation PVC<br />
Conductor material Copper<br />
Ambient temperature (°C) 30<br />
Single-core or multi-core cable Single<br />
Installation method F<br />
Number <strong>of</strong> circuits in close proximity (table G20) 1<br />
Other coefficient 1<br />
Selected cross-sectional area (mm 2 ) 1 x 300<br />
Protective conductor 1 x 150<br />
Length (m) 15<br />
Voltage drop ΔU (%) .38<br />
Voltage drop ΔU total (%) .54<br />
3-phase short-circuit current Ik3 (kA) 20<br />
1-phase-to-earth fault current Id (kA) 13.7<br />
Specific sizing constraint Overloads<br />
G47<br />
© Schneider Electric - all rights reserved
G48<br />
© Schneider Electric - all rights reserved<br />
G - Sizing and protection <strong>of</strong> conductors<br />
8 Worked example <strong>of</strong> cable<br />
calculation<br />
The resistances and the inductive reactances for the six conductors in parallel are,<br />
for a length <strong>of</strong> 5 metres:<br />
for a length <strong>of</strong> 5 metres:<br />
22.5 x 5<br />
R = = 0.20 mΩ (cable resistance: 22.5 22.5 m mΩ.mm<br />
95 x 6<br />
Ω<br />
2 /m)<br />
X = 0.08 x 5 = 0.40 mΩ (cable reactance: 0.08 mΩ/m)<br />
Dimensioning circuit C6<br />
Circuit C6 supplies a 400 kVA 3-phase 400/400 V isolating transformer<br />
3<br />
400. 10<br />
Primary current = = 550 A<br />
420. 3<br />
A single-core cable laid laid on on a a cable cable tray tray (without (without any any other other cable) cable) in an in ambient an ambient air air<br />
temperature <strong>of</strong> 30 °C is proposed. The circuit-breaker is set at 560 A<br />
The method <strong>of</strong> <strong>installation</strong> is characterized by the reference letter F, and the “k”<br />
correcting factors are all equal to 1.<br />
A c.s.a. <strong>of</strong> 240 mm 2 is appropriate.<br />
The resistance and inductive reactance are respectively:<br />
22.5 x 15<br />
R = = 1.4 mΩ<br />
240<br />
X = 0.08 x 15 = 1.2 mΩ<br />
Calculation <strong>of</strong> short-circuit currents for the selection <strong>of</strong> circuit-breakers<br />
Q 1 and Q 6 (see Fig. G67)<br />
Circuits components R (mΩ) X (mΩ) Z (mΩ) Ikmax (kA)<br />
parts<br />
500 MVA at 0.04 0.36<br />
the MV source network<br />
1 MVA transformer 2.2 9.8 10.0 23<br />
Cable C1 0.20 0.4<br />
Sub-total for Q1 2.44 10.6 10.9 23<br />
Busbar B2 3.6 7.2<br />
Cable C6 1.4 1.2<br />
Sub-total for Q6 4.0 8.4 9.3 20<br />
Fig. G67 : Example <strong>of</strong> short-circuit current evaluation<br />
The protective conductor<br />
Thermal requirements: Figures G58 and G59 show that, when using the adiabatic<br />
method the c.s.a. for for the the protective earth earth (PE) (PE) conductor conductor for circuit for C1 circuit will be: C1 will be:<br />
34,800 x 0.2<br />
143<br />
2<br />
= 108 mm<br />
A single 120 mm 2 conductor dimensioned for other reasons mentioned later is<br />
therefore largely sufficient, provided that it also satisfies the requirements for indirectcontact<br />
protection (i.e. that its impedance is sufficiently low).<br />
For the circuit C6, the c.s.a. <strong>of</strong> its PE conductor should be:<br />
29,300 x 0.2<br />
2<br />
= 92 mm<br />
143<br />
In this case a 95 mm2 conductor may be adequate if the indirect-contact protection<br />
conditions are also satisfied.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
G - Sizing and protection <strong>of</strong> conductors<br />
8 Worked example <strong>of</strong> cable<br />
calculation<br />
Protection against indirect-contact hazards<br />
For circuit C6 <strong>of</strong> Figure G65, Figures F45 and F61, or the formula given page F27<br />
may be used for a 3-phase 3-wire circuit.<br />
The maximum permitted length <strong>of</strong> the circuit is given by :<br />
0.8 x 240 x 230 3 x 1,000<br />
Lmax<br />
2 x 22.5 1+ 240<br />
=<br />
= 70 m<br />
⎛ ⎞<br />
x 630 x 11<br />
⎝ 95 ⎠<br />
(The value in the denominator 630 x 11 I=<br />
Im i.e. the current level at which the<br />
instantaneous short-circuit magnetic trip <strong>of</strong> the 630 A circuit-breaker operates).<br />
The length <strong>of</strong> 15 metres is therefore fully protected by “instantaneous” overcurrent<br />
devices.<br />
Voltage drop<br />
From Figure G28 it can be seen that:<br />
b For the cable C1 (6 x 95mm2 per phase)<br />
-1 -1<br />
∆U<br />
0.42 (V A km ) x 1,374 (A) x 0.008 = = 1.54 V<br />
3<br />
∆U%<br />
100 = 1.54 = 0.38%<br />
400 x<br />
b For the circuit C6<br />
∆U<br />
0.21 (V A km ) x 433 (A) x 0.015 -1 -1<br />
= = 1.36 V<br />
3<br />
∆U%<br />
100 = 1.36 = 0.34%<br />
400 x<br />
At the circuit terminals <strong>of</strong> the LV/LV transformer the percentage volt-drop<br />
ΔU% = 0.72%<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
G49<br />
© Schneider Electric - all rights reserved
2<br />
3<br />
4<br />
<strong>Chapter</strong> H<br />
LV switchgear: functions &<br />
selection<br />
Contents<br />
The basic functions <strong>of</strong> LV switchgear H2<br />
1.1 Electrical protection H2<br />
1.2 Isolation H3<br />
1.3 Switchgear control H4<br />
The switchgear H5<br />
2.1 Elementary switching devices H5<br />
2.2 Combined switchgear elements H9<br />
Choice <strong>of</strong> switchgear H 0<br />
3.1 Tabulated functional capabilities H10<br />
3.2 Switchgear selection H10<br />
Circuit-breaker H<br />
4.1 Standards and description H11<br />
4.2 Fundamental characteristics <strong>of</strong> a circuit-breaker H13<br />
4.3 Other characteristics <strong>of</strong> a circuit-breaker H15<br />
4.4 Selection <strong>of</strong> a circuit-breaker H18<br />
4.5 Coordination between circuit-breakers H22<br />
4.6 Discrimination MV/LV in a consumer’s substation H28<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
H<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
H2<br />
H - LV switchgear: functions & selection<br />
The role <strong>of</strong> switchgear is:<br />
b Electrical protection<br />
b Safe isolation from live parts<br />
b Local or remote switching<br />
Electrical protection assures:<br />
b Protection <strong>of</strong> circuit elements against the<br />
thermal and mechanical stresses <strong>of</strong> short-circuit<br />
currents<br />
b Protection <strong>of</strong> persons in the event <strong>of</strong><br />
insulation failure<br />
b Protection <strong>of</strong> appliances and apparatus being<br />
supplied (e.g. motors, etc.)<br />
The basic functions <strong>of</strong><br />
LV switchgear<br />
National and international standards define the manner in which electric circuits <strong>of</strong><br />
LV <strong>installation</strong>s must be realized, and the capabilities and limitations <strong>of</strong> the various<br />
switching devices which are collectively referred to as switchgear.<br />
The main functions <strong>of</strong> switchgear are:<br />
b Electrical protection<br />
b Electrical isolation <strong>of</strong> sections <strong>of</strong> an <strong>installation</strong><br />
b Local or remote switching<br />
These functions are summarized below in Figure H .<br />
Electrical protection at low voltage is (apart from fuses) normally incorporated in<br />
circuit-breakers, in the form <strong>of</strong> thermal-magnetic devices and/or residual-currentoperated<br />
tripping devices (less-commonly, residual voltage- operated devices<br />
- acceptable to, but not recommended by IEC).<br />
In addition to those functions shown in Figure H1, other functions, namely:<br />
b Over-voltage protection<br />
b Under-voltage protection<br />
are provided by specific devices (lightning and various other types <strong>of</strong> voltage-surge<br />
arrester, relays associated with contactors, remotely controlled circuit-breakers, and<br />
with combined circuit-breaker/isolators… and so on)<br />
Electrical protection Isolation Control<br />
against<br />
b Overload currents b Isolation clearly indicated b Functional switching<br />
b Short-circuit currents by an authorized fail-pro<strong>of</strong> b Emergency switching<br />
b Insulation failure mechanical indicator b Emergency stopping<br />
b A gap or interposed insulating b Switching <strong>of</strong>f for<br />
barrier between the open mechanical maintenance<br />
contacts, clearly visible<br />
Fig. H1 : Basic functions <strong>of</strong> LV switchgear<br />
. Electrical protection<br />
The aim is to avoid or to limit the destructive or dangerous consequences <strong>of</strong><br />
excessive (short-circuit) currents, or those due to overloading and insulation failure,<br />
and to separate the defective circuit from the rest <strong>of</strong> the <strong>installation</strong>.<br />
A distinction is made between the protection <strong>of</strong>:<br />
b The elements <strong>of</strong> the <strong>installation</strong> (cables, wires, switchgear…)<br />
b Persons and animals<br />
b Equipment and appliances supplied from the <strong>installation</strong><br />
The protection <strong>of</strong> circuits<br />
v Against overload; a condition <strong>of</strong> excessive current being drawn from a healthy<br />
(unfaulted) <strong>installation</strong><br />
v Against short-circuit currents due to complete failure <strong>of</strong> insulation between<br />
conductors <strong>of</strong> different phases or (in TN systems) between a phase and neutral (or<br />
PE) conductor<br />
Protection in these cases is provided either by fuses or circuit-breaker, in the<br />
distribution board at the origin <strong>of</strong> the final circuit (i.e. the circuit to which the load<br />
is connected). Certain derogations to this rule are authorized in some national<br />
standards, as noted in chapter H1 sub-clause 1.4.<br />
The protection <strong>of</strong> persons<br />
v Against insulation failures. According to the system <strong>of</strong> earthing for the <strong>installation</strong><br />
(TN, TT or IT) the protection will be provided by fuses or circuit-breakers, residual<br />
current devices, and/or permanent monitoring <strong>of</strong> the insulation resistance <strong>of</strong> the<br />
<strong>installation</strong> to earth<br />
The protection <strong>of</strong> electric motors<br />
v Against overheating, due, for example, to long term overloading, stalled rotor,<br />
single-phasing, etc. Thermal relays, specially <strong>design</strong>ed to match the particular<br />
characteristics <strong>of</strong> motors are used.<br />
Such relays may, if required, also protect the motor-circuit cable against overload.<br />
Short-circuit protection is provided either by type aM fuses or by a circuit-breaker<br />
from which the thermal (overload) protective element has been removed, or<br />
otherwise made inoperative.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
H - LV switchgear: functions & selection<br />
A state <strong>of</strong> isolation clearly indicated by an<br />
approved “fail-pro<strong>of</strong>” indicator, or the visible<br />
separation <strong>of</strong> contacts, are both deemed to<br />
satisfy the national standards <strong>of</strong> many countries<br />
(1) the concurrent opening <strong>of</strong> all live conductors, while not<br />
always obligatory, is however, strongly recommended (for<br />
reasons <strong>of</strong> greater safety and facility <strong>of</strong> operation). The neutral<br />
contact opens after the phase contacts, and closes before<br />
them (IEC 60947-1).<br />
The basic functions <strong>of</strong><br />
LV switchgear<br />
.2 Isolation<br />
The aim <strong>of</strong> isolation is to separate a circuit or apparatus (such as a motor, etc.) from<br />
the remainder <strong>of</strong> a system which is energized, in order that personnel may carry out<br />
work on the isolated part in perfect safety.<br />
In principle, all circuits <strong>of</strong> an LV <strong>installation</strong> shall have means to be isolated.<br />
In practice, in order to maintain an optimum continuity <strong>of</strong> service, it is preferred to<br />
provide a means <strong>of</strong> isolation at the origin <strong>of</strong> each circuit.<br />
An isolating device must fulfil the following requirements:<br />
b All poles <strong>of</strong> a circuit, including the neutral (except where the neutral is a PEN<br />
conductor) must open (1)<br />
b It must be provided with a locking system in open position with a key (e.g. by<br />
means <strong>of</strong> a padlock) in order to avoid an unauthorized reclosure by inadvertence<br />
b It must comply with a recognized national or international standard<br />
(e.g. IEC 60947-3) concerning clearance between contacts, creepage distances,<br />
overvoltage withstand capability, etc.:<br />
Other requirements apply:<br />
v Verification that the contacts <strong>of</strong> the isolating device are, in fact, open.<br />
The verification may be:<br />
- Either visual, where the device is suitably <strong>design</strong>ed to allow the contacts to be seen<br />
(some national standards impose this condition for an isolating device located at the<br />
origin <strong>of</strong> a LV <strong>installation</strong> supplied directly from a MV/LV transformer)<br />
- Or mechanical, by means <strong>of</strong> an indicator solidly welded to the operating shaft<br />
<strong>of</strong> the device. In this case the construction <strong>of</strong> the device must be such that, in the<br />
eventuality that the contacts become welded together in the closed position, the<br />
indicator cannot possibly indicate that it is in the open position<br />
v Leakage currents. With the isolating device open, leakage currents between the<br />
open contacts <strong>of</strong> each phase must not exceed:<br />
- 0.5 mA for a new device<br />
- 6.0 mA at the end <strong>of</strong> its useful life<br />
v Voltage-surge withstand capability, across open contacts. The isolating device,<br />
when open must withstand a 1.2/50 μs impulse, having a peak value <strong>of</strong> 6, 8 or 12 kV<br />
according to its service voltage, as shown in Figure H2. The device must satisfy<br />
these conditions for altitudes up to 2,000 metres. Correction factors are given in<br />
IEC 60664-1 for altitudes greater than 2,000 metres.<br />
Consequently, if tests are carried out at sea level, the test values must be increased<br />
by 23% to take into account the effect <strong>of</strong> altitude. See standard IEC 60947.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Service (nominal Impulse withstand<br />
voltage peak voltage category<br />
(V) (for 2,000 metres)<br />
(kV)<br />
III IV<br />
230/400 4 6<br />
400/690 6 8<br />
690/1,000 8 12<br />
Fig. H2 : Peak value <strong>of</strong> impulse voltage according to normal service voltage <strong>of</strong> test specimen.<br />
The degrees III and IV are degrees <strong>of</strong> pollution defined in IEC 60664-1<br />
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H - LV switchgear: functions & selection<br />
Switchgear-control functions allow system<br />
operating personnel to modify a loaded system<br />
at any moment, according to requirements,<br />
and include:<br />
b Functional control (routine switching, etc.)<br />
b Emergency switching<br />
b Maintenance operations on the power system<br />
(1) One break in each phase and (where appropriate) one<br />
break in the neutral.<br />
(2) Taking into account stalled motors.<br />
(3) In a TN schema the PEN conductor must never be<br />
opened, since it functions as a protective earthing wire as well<br />
as the system neutral conductor.<br />
The basic functions <strong>of</strong><br />
LV switchgear<br />
.3 Switchgear control<br />
In broad terms “control” signifies any facility for safely modifying a load-carrying<br />
power system at all levels <strong>of</strong> an <strong>installation</strong>. The operation <strong>of</strong> switchgear is an<br />
important part <strong>of</strong> power-system control.<br />
Functional control<br />
This control relates to all switching operations in normal service conditions for<br />
energizing or de-energizing a part <strong>of</strong> a system or <strong>installation</strong>, or an individual piece<br />
<strong>of</strong> equipment, item <strong>of</strong> plant, etc.<br />
Switchgear intended for such duty must be installed at least:<br />
b At the origin <strong>of</strong> any <strong>installation</strong><br />
b At the final load circuit or circuits (one switch may control several loads)<br />
Marking (<strong>of</strong> the circuits being controlled) must be clear and unambiguous.<br />
In order to provide the maximum flexibility and continuity <strong>of</strong> operation, particularly<br />
where the switching device also constitutes the protection (e.g. a circuit-breaker or<br />
switch-fuse) it is preferable to include a switch at each level <strong>of</strong> distribution, i.e. on<br />
each outgoing way <strong>of</strong> all distribution and subdistribution boards.<br />
The manœuvre may be:<br />
b Either manual (by means <strong>of</strong> an operating lever on the switch) or<br />
b Electric, by push-button on the switch or at a remote location (load-shedding and<br />
reconnection, for example)<br />
These switches operate instantaneously (i.e. with no deliberate delay), and those<br />
that provide protection are invariably omni-polar (1) .<br />
The main circuit-breaker for the entire <strong>installation</strong>, as well as any circuit-breakers<br />
used for change-over (from one source to another) must be omni-polar units.<br />
Emergency switching - emergency stop<br />
An emergency switching is intended to de-energize a live circuit which is, or could<br />
become, dangerous (electric shock or fire).<br />
An emergency stop is intended to halt a movement which has become dangerous.<br />
In the two cases:<br />
b The emergency control device or its means <strong>of</strong> operation (local or at remote<br />
location(s)) such as a large red mushroom-headed emergency-stop pushbutton must<br />
be recognizable and readily accessible, in proximity to any position at which danger<br />
could arise or be seen<br />
b A single action must result in a complete switching-<strong>of</strong>f <strong>of</strong> all live conductors (2) (3)<br />
b A “break glass” emergency switching initiation device is authorized, but in<br />
unmanned <strong>installation</strong>s the re-energizing <strong>of</strong> the circuit can only be achieved by<br />
means <strong>of</strong> a key held by an authorized person<br />
It should be noted that in certain cases, an emergency system <strong>of</strong> braking, may<br />
require that the auxiliary supply to the braking-system circuits be maintained until<br />
final stoppage <strong>of</strong> the machinery.<br />
Switching-<strong>of</strong>f for mechanical maintenance work<br />
This operation assures the stopping <strong>of</strong> a machine and its impossibility to be<br />
inadvertently restarted while mechanical maintenance work is being carried out<br />
on the driven machinery. The shutdown is generally carried out at the functional<br />
switching device, with the use <strong>of</strong> a suitable safety lock and warning notice at the<br />
switch mechanism.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
H - LV switchgear: functions & selection<br />
Fig. H5 : Symbol for a disconnector (or isolator)<br />
Fig. H6 : Symbol for a load-break switch<br />
2 The switchgear<br />
2.1 Elementary switching devices<br />
Disconnector (or isolator) (see Fig. H )<br />
This switch is a manually-operated, lockable, two-position device (open/closed)<br />
which provides safe isolation <strong>of</strong> a circuit when locked in the open position. Its<br />
characteristics are defined in IEC 60947-3. A disconnector is not <strong>design</strong>ed to make<br />
or to break current (1) and no rated values for these functions are given in standards.<br />
It must, however, be capable <strong>of</strong> withstanding the passage <strong>of</strong> short-circuit currents<br />
and is assigned a rated short-time withstand capability, generally for 1 second,<br />
unless otherwise agreed between user and manufacturer. This capability is normally<br />
more than adequate for longer periods <strong>of</strong> (lower-valued) operational overcurrents,<br />
such as those <strong>of</strong> motor-starting. Standardized mechanical-endurance, overvoltage,<br />
and leakage-current tests, must also be satisfied.<br />
Load-breaking switch (see Fig. H6)<br />
This control switch is generally operated manually (but is sometimes provided with<br />
<strong>electrical</strong> tripping for operator convenience) and is a non-automatic two-position<br />
device (open/closed).<br />
It is used to close and open loaded circuits under normal unfaulted circuit conditions.<br />
It does not consequently, provide any protection for the circuit it controls.<br />
IEC standard 60947-3 defines:<br />
b The frequency <strong>of</strong> switch operation (600 close/open cycles per hour maximum)<br />
b Mechanical and <strong>electrical</strong> endurance (generally less than that <strong>of</strong> a contactor)<br />
b Current making and breaking ratings for normal and infrequent situations<br />
When closing a switch to energize a circuit there is always the possibility that<br />
an unsuspected short-circuit exists on the circuit. For this reason, load-break<br />
switches are assigned a fault-current making rating, i.e. successful closure against<br />
the electrodynamic forces <strong>of</strong> short-circuit current is assured. Such switches are<br />
commonly referred to as “fault-make load-break” switches. Upstream protective<br />
devices are relied upon to clear the short-circuit fault<br />
Category AC-23 includes occasional switching <strong>of</strong> individual motors. The switching<br />
<strong>of</strong> capacitors or <strong>of</strong> tungsten filament lamps shall be subject to agreement between<br />
manufacturer and user.<br />
The utilization categories referred to in Figure H7 do not apply to an equipment<br />
normally used to start, accelerate and/or stop individual motors.<br />
Example<br />
A 100 A load-break switch <strong>of</strong> category AC-23 (inductive load) must be able:<br />
b To make a current <strong>of</strong> 10 In (= 1,000 A) at a power factor <strong>of</strong> 0.35 lagging<br />
b To break a current <strong>of</strong> 8 In (= 800 A) at a power factor <strong>of</strong> 0.45 lagging<br />
b To withstand short duration short-circuit currents when closed<br />
Utilization category Typical applications Cos ϕ Making Breaking<br />
Frequent Infrequent current x In current x In<br />
operations operations<br />
AC-20A AC-20B Connecting and disconnecting - - -<br />
under no-load conditions<br />
AC-21A AC-21B Switching <strong>of</strong> resistive loads 0.95 1.5 1.5<br />
including moderate overloads<br />
AC-22A AC-22B Switching <strong>of</strong> mixed resistive 0.65 3 3<br />
and inductive loads, including<br />
moderate overloads<br />
AC-23A AC-23B Switching <strong>of</strong> motor loads or 0.45 for I y 100 A 10 8<br />
other highly inductive loads 0.35 for I > 100 A<br />
Fig. H7 : Utilization categories <strong>of</strong> LV AC switches according to IEC 60947-3<br />
(1) i.e. a LV disconnector is essentially a dead system<br />
switching device to be operated with no voltage on either side<br />
<strong>of</strong> it, particularly when closing, because <strong>of</strong> the possibility <strong>of</strong> an<br />
unsuspected short-circuit on the downstream side. Interlocking<br />
with an upstream switch or circuit-breaker is frequently used.<br />
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H6<br />
H - LV switchgear: functions & selection<br />
Fig. H8 : Symbol for a bistable remote control switch<br />
Control<br />
circuit<br />
Fig. H9 : Symbol for a contactor<br />
Power<br />
circuit<br />
Two classes <strong>of</strong> LV cartridge fuse are very<br />
widely used:<br />
b For domestic and similar <strong>installation</strong>s type gG<br />
b For industrial <strong>installation</strong>s type gG, gM or aM<br />
Fig. H10 : Symbol for fuses<br />
(1) This term is not defined in IEC publications but is commonly<br />
used in some countries.<br />
2 The switchgear<br />
Remote control switch (see Fig. H8)<br />
This device is extensively used in the control <strong>of</strong> lighting circuits where the depression<br />
<strong>of</strong> a pushbutton (at a remote control position) will open an already-closed switch or<br />
close an opened switch in a bistable sequence.<br />
Typical applications are:<br />
b Two-way switching on stairways <strong>of</strong> large buildings<br />
b Stage-lighting schemes<br />
b Factory illumination, etc.<br />
Auxiliary devices are available to provide:<br />
b Remote indication <strong>of</strong> its state at any instant<br />
b Time-delay functions<br />
b Maintained-contact features<br />
Contactor (see Fig. H9)<br />
The contactor is a solenoid-operated switching device which is generally held<br />
closed by (a reduced) current through the closing solenoid (although various<br />
mechanically-latched types exist for specific duties). Contactors are <strong>design</strong>ed to<br />
carry out numerous close/open cycles and are commonly controlled remotely by<br />
on-<strong>of</strong>f pushbuttons. The large number <strong>of</strong> repetitive operating cycles is standardized in<br />
table VIII <strong>of</strong> IEC 60947-4-1 by:<br />
b The operating duration: 8 hours; uninterrupted; intermittent; temporary <strong>of</strong> 3, 10, 30,<br />
60 and 90 minutes<br />
b Utilization category: for example, a contactor <strong>of</strong> category AC3 can be used for the<br />
starting and stopping <strong>of</strong> a cage motor<br />
b The start-stop cycles (1 to 1,200 cyles per hour)<br />
b Mechanical endurance (number <strong>of</strong> <strong>of</strong>f-load manœuvres)<br />
b Electrical endurance (number <strong>of</strong> on-load manœuvres)<br />
b A rated current making and breaking performance according to the category <strong>of</strong><br />
utilization concerned<br />
Example:<br />
A 150 A contactor <strong>of</strong> category AC3 must have a minimum current-breaking capability<br />
<strong>of</strong> 8 In (= 1,200 A) and a minimum current-making rating <strong>of</strong> 10 In (= 1,500 A) at a<br />
power factor (lagging) <strong>of</strong> 0.35.<br />
Discontactor (1)<br />
A contactor equipped with a thermal-type relay for protection against overloading<br />
defines a “discontactor”. Discontactors are used extensively for remote push-button<br />
control <strong>of</strong> lighting circuits, etc., and may also be considered as an essential element<br />
in a motor controller, as noted in sub-clause 2.2. “combined switchgear elements”.<br />
The discontactor is not the equivalent <strong>of</strong> a circuit-breaker, since its short-circuit<br />
current breaking capability is limited to 8 or 10 In. For short-circuit protection<br />
therefore, it is necessary to include either fuses or a circuit-breaker in series with,<br />
and upstream <strong>of</strong>, the discontactor contacts.<br />
Fuses (see Fig. H10)<br />
The first letter indicates the breaking range:<br />
b “g” fuse-links (full-range breaking-capacity fuse-link)<br />
b “a” fuse-links (partial-range breaking-capacity fuse-link)<br />
The second letter indicates the utilization category; this letter defines with accuracy<br />
the time-current characteristics, conventional times and currents, gates.<br />
For example<br />
b “gG” indicates fuse-links with a full-range breaking capacity for general application<br />
b “gM” indicates fuse-links with a full-range breaking capacity for the protection <strong>of</strong><br />
motor circuits<br />
b “aM” indicates fuse-links with a partial range breaking capacity for the protection <strong>of</strong><br />
motor circuits<br />
Fuses exist with and without “fuse-blown” mechanical indicators. Fuses break a<br />
circuit by controlled melting <strong>of</strong> the fuse element when a current exceeds a given<br />
value for a corresponding period <strong>of</strong> time; the current/time relationship being<br />
presented in the form <strong>of</strong> a performance curve for each type <strong>of</strong> fuse. Standards define<br />
two classes <strong>of</strong> fuse:<br />
b Those intended for domestic <strong>installation</strong>s, manufactured in the form <strong>of</strong> a cartridge<br />
for rated currents up to 100 A and <strong>design</strong>ated type gG in IEC 60269-1 and 3<br />
b Those for industrial use, with cartridge types <strong>design</strong>ated gG (general use); and gM<br />
and aM (for motor-circuits) in IEC 60269-1 and 2<br />
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H - LV switchgear: functions & selection<br />
gM fuses require a separate overload relay, as<br />
described in the note at the end <strong>of</strong> sub-clause 2.1.<br />
1 hour<br />
t<br />
Inf I2<br />
Minimum<br />
pre-arcing<br />
time curve<br />
Fuse-blow<br />
curve<br />
Fig. H12 : Zones <strong>of</strong> fusing and non-fusing for gG and gM fuses<br />
(1) Ich for gM fuses<br />
I<br />
2 The switchgear<br />
The main differences between domestic and industrial fuses are the nominal<br />
voltage and current levels (which require much larger physical dimensions) and<br />
their fault-current breaking capabilities. Type gG fuse-links are <strong>of</strong>ten used for the<br />
protection <strong>of</strong> motor circuits, which is possible when their characteristics are capable<br />
<strong>of</strong> withstanding the motor-starting current without deterioration.<br />
A more recent development has been the adoption by the IEC <strong>of</strong> a fuse-type gM for<br />
motor protection, <strong>design</strong>ed to cover starting, and short-circuit conditions. This type <strong>of</strong><br />
fuse is more popular in some countries than in others, but at the present time the<br />
aM fuse in combination with a thermal overload relay is more-widely used.<br />
A gM fuse-link, which has a dual rating is characterized by two current values. The<br />
first value In denotes both the rated current <strong>of</strong> the fuse-link and the rated current <strong>of</strong><br />
the fuseholder; the second value Ich denotes the time-current characteristic <strong>of</strong> the<br />
fuse-link as defined by the gates in Tables II, III and VI <strong>of</strong> IEC 60269-1.<br />
These two ratings are separated by a letter which defines the applications.<br />
For example: In M Ich denotes a fuse intended to be used for protection <strong>of</strong><br />
motor circuits and having the characteristic G. The first value In corresponds to<br />
the maximum continuous current for the whole fuse and the second value Ich<br />
corresponds to the G characteristic <strong>of</strong> the fuse link. For further details see note at the<br />
end <strong>of</strong> sub-clause 2.1.<br />
An aM fuse-link is characterized by one current value In and time-current<br />
characteristic as shown in Figure H14 next page.<br />
Important: Some national standards use a gI (industrial) type fuse, similar in all main<br />
essentails to type gG fuses.<br />
Type gI fuses should never be used, however, in domestic and similar <strong>installation</strong>s.<br />
Fusing zones - conventional currents<br />
The conditions <strong>of</strong> fusing (melting) <strong>of</strong> a fuse are defined by standards, according to<br />
their class.<br />
Class gG fuses<br />
These fuses provide protection against overloads and short-circuits.<br />
Conventional non-fusing and fusing currents are standardized, as shown in<br />
Figure H12 and in Figure H13.<br />
b The conventional non-fusing current Inf is the value <strong>of</strong> current that the fusible<br />
element can carry for a specified time without melting.<br />
Example: A 32 A fuse carrying a current <strong>of</strong> 1.25 In (i.e. 40 A) must not melt in less<br />
than one hour (table H13)<br />
b The conventional fusing current If (= I2 in Fig. H12) is the value <strong>of</strong> current which<br />
will cause melting <strong>of</strong> the fusible element before the expiration <strong>of</strong> the specified time.<br />
Example: A 32 A fuse carrying a current <strong>of</strong> 1.6 In (i.e. 52.1 A) must melt in one hour<br />
or less<br />
IEC 60269-1 standardized tests require that a fuse-operating characteristic lies<br />
between the two limiting curves (shown in Figure H12) for the particular fuse under<br />
test. This means that two fuses which satisfy the test can have significantly different<br />
operating times at low levels <strong>of</strong> overloading.<br />
Rated current (1) Conventional non- Conventional Conventional<br />
In (A) fusing current fusing current time (h)<br />
Inf I2<br />
In y 4 A 1.5 In 2.1 In 1<br />
4 < In < 16 A 1.5 In 1.9 In 1<br />
16 < In y 63 A 1.25 In 1.6 In 1<br />
63 < In y 160 A 1.25 In 1.6 In 2<br />
160 < In y 400 A 1.25 In 1.6 In 3<br />
400 < In 1.25 In 1.6 In 4<br />
Fig. H13 : Zones <strong>of</strong> fusing and non-fusing for LV types gG and gM class fuses (IEC 60269-1<br />
and 60269-2-1)<br />
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H - LV switchgear: functions & selection<br />
Class aM fuses protect against short-circuit<br />
currents only, and must always be associated<br />
with another device which protects against<br />
overload<br />
4 I n<br />
x I n<br />
Fig. H14 : Standardized zones <strong>of</strong> fusing for type aM fuses (all<br />
current ratings)<br />
I<br />
t<br />
Tf Ta<br />
Ttc<br />
0.005 s<br />
0.02 s<br />
0.01 s<br />
Tf: Fuse pre-arc fusing time<br />
Ta: Arcing time<br />
Ttc: Total fault-clearance time<br />
Fig. H15 : Current limitation by a fuse<br />
Mini m um<br />
pre-arcing<br />
time cu r v e<br />
Fuse- b l o wn<br />
cu r v e<br />
Prospective<br />
fault-current peak<br />
rms value <strong>of</strong> the AC<br />
component <strong>of</strong> the<br />
prospective fault curent<br />
Current peak<br />
limited by the fuse<br />
(1) For currents exceeding a certain level, depending on the<br />
fuse nominal current rating, as shown below in Figure H16.<br />
t<br />
2 The switchgear<br />
b The two examples given above for a 32 A fuse, together with the foregoing notes<br />
on standard test requirements, explain why these fuses have a poor performance in<br />
the low overload range<br />
b It is therefore necessary to install a cable larger in ampacity than that normally<br />
required for a circuit, in order to avoid the consequences <strong>of</strong> possible long term<br />
overloading (60% overload for up to one hour in the worst case)<br />
By way <strong>of</strong> comparison, a circuit-breaker <strong>of</strong> similar current rating:<br />
b Which passes 1.05 In must not trip in less than one hour; and<br />
b When passing 1.25 In it must trip in one hour, or less (25% overload for up to one<br />
hour in the worst case)<br />
Class aM (motor) fuses<br />
These fuses afford protection against short-circuit currents only and must necessarily<br />
be associated with other switchgear (such as discontactors or circuit-breakers) in<br />
order to ensure overload protection < 4 In. They are not therefore autonomous. Since<br />
aM fuses are not intended to protect against low values <strong>of</strong> overload current, no levels<br />
<strong>of</strong> conventional non-fusing and fusing currents are fixed. The characteristic curves for<br />
testing these fuses are given for values <strong>of</strong> fault current exceeding approximately 4 In<br />
(see Fig. H14), and fuses tested to IEC 60269 must give operating curves which fall<br />
within the shaded area.<br />
Note: the small “arrowheads” in the diagram indicate the current/time “gate” values<br />
for the different fuses to be tested (IEC 60269).<br />
Rated short-circuit breaking currents<br />
A characteristic <strong>of</strong> modern cartridge fuses is that, owing to the rapidity <strong>of</strong> fusion<br />
in the case <strong>of</strong> high short-circuit current levels (1) , a current cut-<strong>of</strong>f begins before<br />
the occurrence <strong>of</strong> the first major peak, so that the fault current never reaches its<br />
prospective peak value (see Fig. H1 ).<br />
This limitation <strong>of</strong> current reduces significantly the thermal and dynamic stresses<br />
which would otherwise occur, thereby minimizing danger and damage at the fault<br />
position. The rated short-circuit breaking current <strong>of</strong> the fuse is therefore based on the<br />
rms value <strong>of</strong> the AC component <strong>of</strong> the prospective fault current.<br />
No short-circuit current-making rating is assigned to fuses.<br />
Reminder<br />
Short-circuit currents initially contain DC components, the magnitude and duration <strong>of</strong><br />
which depend on the XL/R ratio <strong>of</strong> the fault current loop.<br />
Close to the source (MV/LV transformer) the relationship Ipeak / Irms (<strong>of</strong><br />
AC component) immediately following the instant <strong>of</strong> fault, can be as high as 2.5<br />
(standardized by IEC, and shown in Figure H16 next page).<br />
At lower levels <strong>of</strong> distribution in an <strong>installation</strong>, as previously noted, XL is small<br />
compared with R and so for final circuits Ipeak / Irms ~ 1.41, a condition which<br />
corresponds with Figure H15.<br />
The peak-current-limitation effect occurs only when the prospective rms<br />
AC component <strong>of</strong> fault current attains a certain level. For example, in the Figure H16<br />
graph, the 100 A fuse will begin to cut <strong>of</strong>f the peak at a prospective fault current<br />
(rms) <strong>of</strong> 2 kA (a). The same fuse for a condition <strong>of</strong> 20 kA rms prospective current<br />
will limit the peak current to 10 kA (b). Without a current-limiting fuse the peak<br />
current could attain 50 kA (c) in this particular case. As already mentioned, at lower<br />
distribution levels in an <strong>installation</strong>, R greatly predominates XL, and fault levels are<br />
generally low. This means that the level <strong>of</strong> fault current may not attain values high<br />
enough to cause peak current limitation. On the other hand, the DC transients (in this<br />
case) have an insignificant effect on the magnitude <strong>of</strong> the current peak, as previously<br />
mentioned.<br />
Note: On gM fuse ratings<br />
A gM type fuse is essentially a gG fuse, the fusible element <strong>of</strong> which corresponds to<br />
the current value Ich (ch = characteristic) which may be, for example, 63 A. This is<br />
the IEC testing value, so that its time/ current characteristic is identical to that <strong>of</strong> a<br />
63 A gG fuse.<br />
This value (63 A) is selected to withstand the high starting currents <strong>of</strong> a motor, the<br />
steady state operating current (In) <strong>of</strong> which may be in the 10-20 A range.<br />
This means that a physically smaller fuse barrel and metallic parts can be used,<br />
since the heat dissipation required in normal service is related to the lower figures<br />
(10-20 A). A standard gM fuse, suitable for this situation would be <strong>design</strong>ated 32M63<br />
(i.e. In M Ich).<br />
The first current rating In concerns the steady-load thermal performance <strong>of</strong> the<br />
fuselink, while the second current rating (Ich) relates to its (short-time) startingcurrent<br />
performance. It is evident that, although suitable for short-circuit protection,<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
H - LV switchgear: functions & selection<br />
Prospective fault<br />
current (kA) peak<br />
100<br />
50<br />
20<br />
10<br />
5<br />
2<br />
(a)<br />
Maximum possible current<br />
peak characteristic<br />
i.e. 2.5 Irms (IEC)<br />
Fig. H17 : Symbol for an automatic tripping switch-fuse<br />
(c)<br />
(b)<br />
160A<br />
100A<br />
50A<br />
1<br />
1 2 5 10 20 50 100<br />
AC component <strong>of</strong> prospective<br />
fault current (kA) rms<br />
Fig. H16 : Limited peak current versus prospective rms values<br />
<strong>of</strong> the AC component <strong>of</strong> fault current for LV fuses<br />
Fig. H18 : Symbol for a non-automatic fuse-switch<br />
Nominal<br />
fuse<br />
ratings<br />
Peak current<br />
cut-<strong>of</strong>f<br />
characteristic<br />
curves<br />
Fig. H20 : Symbol for a fuse disconnector + discontactor<br />
Fig. H21 : Symbol for a fuse-switch disconnector + discontactor<br />
2 The switchgear<br />
overload protection for the motor is not provided by the fuse, and so a separate<br />
thermal-type relay is always necessary when using gM fuses. The only advantage<br />
<strong>of</strong>fered by gM fuses, therefore, when compared with aM fuses, are reduced physical<br />
dimensions and slightly lower cost.<br />
2.2 Combined switchgear elements<br />
Single units <strong>of</strong> switchgear do not, in general, fulfil all the requirements <strong>of</strong> the three<br />
basic functions, viz: Protection, control and isolation.<br />
Where the <strong>installation</strong> <strong>of</strong> a circuit-breaker is not appropriate (notably where the<br />
switching rate is high, over extended periods) combinations <strong>of</strong> units specifically<br />
<strong>design</strong>ed for such a performance are employed. The most commonly-used<br />
combinations are described below.<br />
Switch and fuse combinations<br />
Two cases are distinguished:<br />
b The type in which the operation <strong>of</strong> one (or more) fuse(s) causes the switch to open.<br />
This is achieved by the use <strong>of</strong> fuses fitted with striker pins, and a system <strong>of</strong> switch<br />
tripping springs and toggle mechanisms (see Fig. H17)<br />
b The type in which a non-automatic switch is associated with a set <strong>of</strong> fuses in a<br />
common enclosure.<br />
In some countries, and in IEC 60947-3, the terms “switch-fuse” and “fuse-switch”<br />
have specific meanings, viz:<br />
v A switch-fuse comprises a switch (generally 2 breaks per pole) on the upstream<br />
side <strong>of</strong> three fixed fuse-bases, into which the fuse carriers are inserted (see Fig. H18)<br />
v A fuse-switch consists <strong>of</strong> three switch blades each constituting a double-break per<br />
phase.<br />
These blades are not continuous throughout their length, but each has a gap in the<br />
centre which is bridged by the fuse cartridge. Some <strong>design</strong>s have only a single break<br />
per phase, as shown in Figure H19.<br />
Fig. H19 : Symbol for a non-automatic switch-fuse<br />
The current range for these devices is limited to 100 A maximum at 400 V 3-phase,<br />
while their principal use is in domestic and similar <strong>installation</strong>s. To avoid confusion<br />
between the first group (i.e. automatic tripping) and the second group, the term<br />
“switch-fuse” should be qualified by the adjectives “automatic” or “non-automatic”.<br />
Fuse – disconnector + discontactor<br />
Fuse - switch-disconnector + discontactor<br />
As previously mentioned, a discontactor does not provide protection against shortcircuit<br />
faults. It is necessary, therefore, to add fuses (generally <strong>of</strong> type aM) to perform<br />
this function. The combination is used mainly for motor control circuits, where the<br />
disconnector or switch-disconnector allows safe operations such as:<br />
b The changing <strong>of</strong> fuse links (with the circuit isolated)<br />
b Work on the circuit downstream <strong>of</strong> the discontactor (risk <strong>of</strong> remote closure <strong>of</strong> the<br />
discontactor)<br />
The fuse-disconnector must be interlocked with the discontactor such that no opening<br />
or closing manœuvre <strong>of</strong> the fuse disconnector is possible unless the discontactor is<br />
open ( Figure H20), since the fuse disconnector has no load-switching capability.<br />
A fuse-switch-disconnector (evidently) requires no interlocking (Figure H21).<br />
The switch must be <strong>of</strong> class AC22 or AC23 if the circuit supplies a motor.<br />
Circuit-breaker + contactor<br />
Circuit-breaker + discontactor<br />
These combinations are used in remotely controlled distribution systems in which the<br />
rate <strong>of</strong> switching is high, or for control and protection <strong>of</strong> a circuit supplying motors.<br />
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H - LV switchgear: functions & selection<br />
3 Choice <strong>of</strong> switchgear<br />
3.1 Tabulated functional capabilities<br />
After having studied the basic functions <strong>of</strong> LV switchgear (clause 1, Figure H1) and<br />
the different components <strong>of</strong> switchgear (clause 2), Figure H22 summarizes the<br />
capabilities <strong>of</strong> the various components to perform the basic functions.<br />
Isolation Control Electrical protection<br />
Switchgear Functional Emergency Emergency Switching for Overload Short-circuit Electric<br />
item switching stop mechanical shock<br />
(mechanical) maintenance<br />
Isolator (or b<br />
disconnector) (4)<br />
Switch (5) b b b (1) b (1) (2) b<br />
Residual b b b (1) b (1) (2) b b<br />
device<br />
(RCCB) (5)<br />
Switch- b b b (1) b (1) (2) b<br />
disconnector<br />
Contactor b b (1) b (1) (2) b b (3)<br />
Remote control b b (1) b<br />
switch<br />
Fuse b b b<br />
Circuit b b (1) b (1) (2) b b b<br />
breaker (5)<br />
Circuit-breaker b b b (1) b (1) (2) b b b<br />
disconnector (5)<br />
Residual b b b (1) b (1) (2) b b b b<br />
and overcurrent<br />
circuit-breaker<br />
(RCBO) (5)<br />
Point <strong>of</strong> Origin <strong>of</strong> each All points where, In general at the At the supply At the supply Origin <strong>of</strong> each Origin <strong>of</strong> each Origin <strong>of</strong> circuits<br />
<strong>installation</strong> circuit for operational incoming circuit point to each point to each circuit circuit where the<br />
(general reasons it may to every machine machine earthing system<br />
principle) be necessary distribution and/or on the is appropriate<br />
to stop the board machine TN-S, IT, TT<br />
process concerned<br />
(1) Where cut-<strong>of</strong>f <strong>of</strong> all active conductors is provided<br />
(2) It may be necessary to maintain supply to a braking system<br />
(3) If it is associated with a thermal relay (the combination is commonly referred to as a “discontactor”)<br />
(4) In certain countries a disconnector with visible contacts is mandatory at the origin <strong>of</strong> a LV <strong>installation</strong> supplied directly from a MV/LV transformer<br />
(5) Certain items <strong>of</strong> switchgear are suitable for isolation duties (e.g. RCCBs according to IEC 61008) without being explicitly marked as such<br />
Fig. H22 : Functions fulfilled by different items <strong>of</strong> switchgear<br />
3.2 Switchgear selection<br />
S<strong>of</strong>tware is being used more and more in the field <strong>of</strong> optimal selection <strong>of</strong> switchgear.<br />
Each circuit is considered one at a time, and a list is drawn up <strong>of</strong> the required<br />
protection functions and exploitation <strong>of</strong> the <strong>installation</strong>, among those mentioned in<br />
Figure H22 and summarized in Figure H1.<br />
A number <strong>of</strong> switchgear combinations are studied and compared with each other<br />
against relevant criteria, with the aim <strong>of</strong> achieving:<br />
b Satisfactory performance<br />
b Compatibility among the individual items; from the rated current In to the fault-level<br />
rating Icu<br />
b Compatibility with upstream switchgear or taking into account its contribution<br />
b Conformity with all regulations and specifications concerning safe and reliable<br />
circuit performance<br />
In order to determine the number <strong>of</strong> poles for an item <strong>of</strong> switchgear, reference is<br />
made to chapter G, clause 7 Fig. G64. Multifunction switchgear, initially more costly,<br />
reduces <strong>installation</strong> costs and problems <strong>of</strong> <strong>installation</strong> or exploitation. It is <strong>of</strong>ten found<br />
that such switchgear provides the best solution.<br />
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H - LV switchgear: functions & selection<br />
The circuit-breaker/disconnector fulfills all <strong>of</strong> the<br />
basic switchgear functions, while, by means <strong>of</strong><br />
accessories, numerous other possibilities exist<br />
Industrial circuit-breakers must comply with<br />
IEC 60947-1 and 60947-2 or other equivalent<br />
standards.<br />
Domestic-type circuit-breakers must comply with<br />
IEC standard 60898, or an equivalent national<br />
standard<br />
Fig. H24 : Main parts <strong>of</strong> a circuit-breaker<br />
Power circuit terminals<br />
Contacts and arc-diving<br />
chamber<br />
Fool-pro<strong>of</strong> mechanical<br />
indicator<br />
Latching mechanism<br />
Trip mechanism and<br />
protective devices<br />
4 Circuit-breaker<br />
As shown in Figure H23 the circuit-breaker/ disconnector is the only item <strong>of</strong><br />
switchgear capable <strong>of</strong> simultaneously satisfying all the basic functions necessary in<br />
an <strong>electrical</strong> <strong>installation</strong>.<br />
Moreover, it can, by means <strong>of</strong> auxiliary units, provide a wide range <strong>of</strong> other functions,<br />
for example: indication (on-<strong>of</strong>f - tripped on fault); undervoltage tripping; remote<br />
control… etc. These features make a circuit-breaker/ disconnector the basic unit <strong>of</strong><br />
switchgear for any <strong>electrical</strong> <strong>installation</strong>.<br />
Functions Possible conditions<br />
Isolation b<br />
Control Functional b<br />
Emergency switching b (With the possibility <strong>of</strong> a tripping<br />
coil for remote control)<br />
Switching-<strong>of</strong>f for mechanical b<br />
maintenance<br />
Protection Overload b<br />
Short-circuit b<br />
Insulation fault b (With differential-current relay)<br />
Undervoltage b (With undervoltage-trip coil)<br />
Remote control b Added or incorporated<br />
Indication and measurement b (<strong>General</strong>ly optional with an<br />
electronic tripping device)<br />
Fig. H23 : Functions performed by a circuit-breaker/disconnector<br />
4.1 Standards and description<br />
Standards<br />
For industrial LV <strong>installation</strong>s the relevant IEC standards are, or are due to be:<br />
b 60947-1: general <strong>rules</strong><br />
b 60947-2: part 2: circuit-breakers<br />
b 60947-3: part 3: switches, disconnectors, switch-disconnectors and fuse<br />
combination units<br />
b 60947-4: part 4: contactors and motor starters<br />
b 60947-5: part 5: control-circuit devices and switching elements<br />
b 60947-6: part 6: multiple function switching devices<br />
b 60947-7: part 7: ancillary equipment<br />
For domestic and similar LV <strong>installation</strong>s, the appropriate standard is IEC 60898, or<br />
an equivalent national standard.<br />
Description<br />
Figure H24 shows schematically the main parts <strong>of</strong> a LV circuit-breaker and its four<br />
essential functions:<br />
b The circuit-breaking components, comprising the fixed and moving contacts and<br />
the arc-dividing chamber<br />
b The latching mechanism which becomes unlatched by the tripping device on<br />
detection <strong>of</strong> abnormal current conditions<br />
This mechanism is also linked to the operation handle <strong>of</strong> the breaker.<br />
b A trip-mechanism actuating device:<br />
v Either: a thermal-magnetic device, in which a thermally-operated bi-metal strip<br />
detects an overload condition, while an electromagnetic striker pin operates at<br />
current levels reached in short-circuit conditions, or<br />
v An electronic relay operated from current transformers, one <strong>of</strong> which is installed on<br />
each phase<br />
b A space allocated to the several types <strong>of</strong> terminal currently used for the main<br />
power circuit conductors<br />
Domestic circuit-breakers (see Fig. H25 next page) complying with IEC 60898 and<br />
similar national standards perform the basic functions <strong>of</strong>:<br />
b Isolation<br />
b Protection against overcurrent<br />
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H - LV switchgear: functions & selection<br />
Fig. H25 : Domestic-type circuit-breaker providing overcurrent<br />
protection and circuit isolation features<br />
Fig. H26 : Domestic-type circuit-breaker as above (Fig. H25)<br />
with incorparated protection against electric shocks<br />
OF1 SD SDE OF2<br />
OF2<br />
SDE<br />
SD<br />
OF1<br />
Fig. H28 : Example <strong>of</strong> a modular (Compact NS) industrial type<br />
<strong>of</strong> circuit-breaker capable <strong>of</strong> numerous auxiliary functions<br />
4 Circuit-breaker<br />
Some models can be adapted to provide sensitive detection (30 mA) <strong>of</strong> earthleakage<br />
current with CB tripping, by the addition <strong>of</strong> a modular block, while other<br />
models (RCBOs, complying with IEC 61009 and CBRs complying with IEC 60947-2<br />
Annex B) have this residual current feature incorporated as shown in Figure H26.<br />
Apart from the above-mentioned functions further features can be associated with<br />
the basic circuit-breaker by means <strong>of</strong> additional modules, as shown in Figure H27;<br />
notably remote control and indication (on-<strong>of</strong>f-fault).<br />
5<br />
4<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
3<br />
2<br />
O-OFF O-OFF - O-OFF -<br />
Fig. H27 : “Multi 9” system <strong>of</strong> LV modular switchgear components<br />
Moulded-case type industrial circuit-breakers complying with IEC 60947-2 are now<br />
available, which, by means <strong>of</strong> associated adaptable blocks provide a similar range <strong>of</strong><br />
auxiliary functions to those described above (see Figure H28).<br />
Heavy-duty industrial circuit-breakers <strong>of</strong> large current ratings, complying with<br />
IEC 60947-2, have numerous built-in communication and electronic functions<br />
(see Figure H29).<br />
In addition to the protection functions, the Micrologic unit provides optimized<br />
functions such as measurement (including power quality functions), diagnosis,<br />
communication, control and monitoring.<br />
Fig. H29 : Examples <strong>of</strong> heavy-duty industrial circuit-breakers. The “Masterpact” provides many<br />
automation features in its “Micrologic” tripping module<br />
1
H - LV switchgear: functions & selection<br />
0.4 In<br />
Adjustment<br />
range<br />
Overload trip<br />
current setting<br />
Ir<br />
Rated current <strong>of</strong><br />
the tripping unit<br />
160 A 360 A 400 A 630 A<br />
(1) Current-level setting values which refer to the currentoperated<br />
thermal and “instantaneous” magnetic tripping<br />
devices for over-load and short-circuit protection.<br />
In<br />
Circuit breaker<br />
frame-size rating<br />
Fig. H30 : Example <strong>of</strong> a NS630N circuit-breaker equipped with<br />
a STR23SE trip unit adjusted to 0.9, to give Ir = 360 A<br />
4 Circuit-breaker<br />
4.2 Fundamental characteristics <strong>of</strong> a circuit-breaker<br />
The fundamental characteristics <strong>of</strong> a circuit-breaker are:<br />
b Its rated voltage Ue<br />
b Its rated current In<br />
b Its tripping-current-level adjustment ranges for overload protection (Ir (1) or Irth (1) )<br />
and for short-circuit protection (Im) (1)<br />
b Its short-circuit current breaking rating (Icu for industrial CBs; Icn for domestic-<br />
type CBs).<br />
Rated operational voltage (Ue)<br />
This is the voltage at which the circuit-breaker has been <strong>design</strong>ed to operate, in<br />
normal (undisturbed) conditions.<br />
Other values <strong>of</strong> voltage are also assigned to the circuit-breaker, corresponding to<br />
disturbed conditions, as noted in sub-clause 4.3.<br />
Rated current (In)<br />
This is the maximum value <strong>of</strong> current that a circuit-breaker, fitted with a specified<br />
overcurrent tripping relay, can carry indefinitely at an ambient temperature stated by<br />
the manufacturer, without exceeding the specified temperature limits <strong>of</strong> the current<br />
carrying parts.<br />
Example<br />
A circuit-breaker rated at In = 125 A for an ambient temperature <strong>of</strong> 40 °C will be<br />
equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). The<br />
same circuit-breaker can be used at higher values <strong>of</strong> ambient temperature however,<br />
if suitably “derated”. Thus, the circuit-breaker in an ambient temperature <strong>of</strong> 50 °C<br />
could carry only 117 A indefinitely, or again, only 109 A at 60 °C, while complying<br />
with the specified temperature limit.<br />
Derating a circuit-breaker is achieved therefore, by reducing the trip-current setting<br />
<strong>of</strong> its overload relay, and marking the CB accordingly. The use <strong>of</strong> an electronic-type<br />
<strong>of</strong> tripping unit, <strong>design</strong>ed to withstand high temperatures, allows circuit-breakers<br />
(derated as described) to operate at 60 °C (or even at 70 °C) ambient.<br />
Note: In for circuit-breakers (in IEC 60947-2) is equal to Iu for switchgear generally,<br />
Iu being the rated uninterrupted current.<br />
Frame-size rating<br />
A circuit-breaker which can be fitted with overcurrent tripping units <strong>of</strong> different current<br />
level-setting ranges, is assigned a rating which corresponds to the highest currentlevel-setting<br />
tripping unit that can be fitted.<br />
Example<br />
A NS630N circuit-breaker can be equipped with 4 electronic trip units from 150 A to<br />
630 A. The size <strong>of</strong> the circuit-breaker is 630 A.<br />
Overload relay trip-current setting (Irth or Ir)<br />
Apart from small circuit-breakers which are very easily replaced, industrial circuitbreakers<br />
are equipped with removable, i.e. exchangeable, overcurrent-trip relays.<br />
Moreover, in order to adapt a circuit-breaker to the requirements <strong>of</strong> the circuit<br />
it controls, and to avoid the need to install over-sized cables, the trip relays are<br />
generally adjustable. The trip-current setting Ir or Irth (both <strong>design</strong>ations are<br />
in common use) is the current above which the circuit-breaker will trip. It also<br />
represents the maximum current that the circuit-breaker can carry without tripping.<br />
That value must be greater than the maximum load current IB, but less than the<br />
maximum current permitted in the circuit Iz (see chapter G, sub-clause 1.3).<br />
The thermal-trip relays are generally adjustable from 0.7 to 1.0 times In, but when<br />
electronic devices are used for this duty, the adjustment range is greater; typically 0.4<br />
to 1 times In.<br />
Example (see Fig. H30)<br />
A NS630N circuit-breaker equipped with a 400 A STR23SE overcurrent trip relay, set<br />
at 0.9, will have a trip-current setting:<br />
Ir = 400 x 0.9 = 360 A<br />
Note: For circuit-breakers equipped with non-adjustable overcurrent-trip relays,<br />
Ir = In. Example: for C60N 20 A circuit-breaker, Ir = In = 20 A.<br />
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H - LV switchgear: functions & selection<br />
Short-circuit relay trip-current setting (Im)<br />
Fig. H31 : Tripping-current ranges <strong>of</strong> overload and short-circuit protective devices for LV circuit-breakers<br />
t (s )<br />
Ir Im Icu<br />
I(A<br />
Fig. H32 : Performance curve <strong>of</strong> a circuit-breaker thermalmagnetic<br />
protective scheme<br />
4 Circuit-breaker<br />
Type <strong>of</strong> Overload Short-circuit protection<br />
protective protection<br />
relay<br />
Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended to<br />
trip the circuit-breaker rapidly on the occurrence <strong>of</strong> high values <strong>of</strong> fault current. Their<br />
tripping threshold Im is:<br />
b Either fixed by standards for domestic type CBs, e.g. IEC 60898, or,<br />
b Indicated by the manufacturer for industrial type CBs according to related<br />
standards, notably IEC 60947-2.<br />
For the latter circuit-breakers there exists a wide variety <strong>of</strong> tripping devices which<br />
allow a user to adapt the protective performance <strong>of</strong> the circuit-breaker to the<br />
particular requirements <strong>of</strong> a load (see Fig. H31, Fig. H32 and Fig. H33).<br />
Domestic Thermal- Ir = In Low setting Standard setting High setting circuit<br />
breakers magnetic type B type C type D<br />
IEC 60898 3 In y Im y 5 In 5 In y Im y 10 In 10 In y Im y 20 In (1)<br />
Modular Thermal- Ir = In Low setting Standard setting High setting<br />
industrial (2) magnetic fixed type B or Z type C type D or K<br />
circuit-breakers 3.2 In y fixed y 4.8 In 7 In y fixed y 10 In 10 In y fixed y 14 In<br />
Industrial (2) Thermal- Ir = In fixed Fixed: Im = 7 to 10 In<br />
circuit-breakers magnetic Adjustable: Adjustable:<br />
IEC 60947-2 0.7 In y Ir y In - Low setting : 2 to 5 In<br />
- Standard setting: 5 to 10 In<br />
Electronic Long delay Short-delay, adjustable<br />
0.4 In y Ir y In 1.5 Ir y Im y 10 Ir<br />
Instantaneous (I) fixed<br />
I = 12 to 15 In<br />
(1) 50 In in IEC 60898, which is considered to be unrealistically high by most European manufacturers (Merlin Gerin = 10 to 14 In).<br />
(2) For industrial use, IEC standards do not specify values. The above values are given only as being those in common use.<br />
t (s )<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Ir Im Ii Icu<br />
Fig. H33 : Performance curve <strong>of</strong> a circuit-breaker electronic protective scheme<br />
I(A<br />
Ir: Overload (thermal or long-delay) relay trip-current<br />
setting<br />
Im: Short-circuit (magnetic or short-delay) relay tripcurrent<br />
setting<br />
Ii: Short-circuit instantaneous relay trip-current setting.<br />
Icu: Breaking capacity
H - LV switchgear: functions & selection<br />
The short-circuit current-breaking performance<br />
<strong>of</strong> a LV circuit-breaker is related (approximately)<br />
to the cos ϕ <strong>of</strong> the fault-current loop. Standard<br />
values for this relationship have been<br />
established in some standards<br />
Familiarity with the following less-important<br />
characteristics <strong>of</strong> LV circuit-breakers is,<br />
however, <strong>of</strong>ten necessary when making a final<br />
choice.<br />
4 Circuit-breaker<br />
Isolating feature<br />
A circuit-breaker is suitable for isolating a circuit if it fulfills all the conditions<br />
prescribed for a disconnector (at its rated voltage) in the relevant standard (see<br />
sub-clause 1.2). In such a case it is referred to as a circuit-breaker-disconnector and<br />
marked on its front face with the symbol<br />
All Multi 9, Compact NS and Masterpact LV switchgear <strong>of</strong> Merlin Gerin manufacture<br />
is in this category.<br />
Rated short-circuit breaking capacity (Icu or Icn)<br />
The short-circuit current-breaking rating <strong>of</strong> a CB is the highest (prospective) value<br />
<strong>of</strong> current that the CB is capable <strong>of</strong> breaking without being damaged. The value<br />
<strong>of</strong> current quoted in the standards is the rms value <strong>of</strong> the AC component <strong>of</strong> the<br />
fault current, i.e. the DC transient component (which is always present in the worst<br />
possible case <strong>of</strong> short-circuit) is assumed to be zero for calculating the standardized<br />
value. This rated value (Icu) for industrial CBs and (Icn) for domestic-type CBs is<br />
normally given in kA rms.<br />
Icu (rated ultimate s.c. breaking capacity) and Ics (rated service s.c. breaking<br />
capacity) are defined in IEC 60947-2 together with a table relating Ics with Icu for<br />
different categories <strong>of</strong> utilization A (instantaneous tripping) and B (time-delayed<br />
tripping) as discussed in subclause 4.3.<br />
Tests for proving the rated s.c. breaking capacities <strong>of</strong> CBs are governed by<br />
standards, and include:<br />
b Operating sequences, comprising a succession <strong>of</strong> operations, i.e. closing and<br />
opening on short-circuit<br />
b Current and voltage phase displacement. When the current is in phase with the<br />
supply voltage (cos ϕ for the circuit = 1), interruption <strong>of</strong> the current is easier than<br />
that at any other power factor. Breaking a current at low lagging values <strong>of</strong> cos ϕ is<br />
considerably more difficult to achieve; a zero power-factor circuit being (theoretically)<br />
the most onerous case.<br />
In practice, all power-system short-circuit fault currents are (more or less) at lagging<br />
power factors, and standards are based on values commonly considered to be<br />
representative <strong>of</strong> the majority <strong>of</strong> power systems. In general, the greater the level <strong>of</strong><br />
fault current (at a given voltage), the lower the power factor <strong>of</strong> the fault-current loop,<br />
for example, close to generators or large transformers.<br />
Figure H34 below extracted from IEC 60947-2 relates standardized values <strong>of</strong> cos ϕ<br />
to industrial circuit-breakers according to their rated Icu.<br />
b Following an open - time delay - close/open sequence to test the Icu capacity <strong>of</strong> a<br />
CB, further tests are made to ensure that:<br />
v The dielectric withstand capability<br />
v The disconnection (isolation) performance and<br />
v The correct operation <strong>of</strong> the overload protection<br />
have not been impaired by the test.<br />
4.3 Other characteristics <strong>of</strong> a circuit-breaker<br />
Rated insulation voltage (Ui)<br />
This is the value <strong>of</strong> voltage to which the dielectric tests voltage (generally greater<br />
than 2 Ui) and creepage distances are referred to.<br />
The maximum value <strong>of</strong> rated operational voltage must never exceed that <strong>of</strong> the rated<br />
insulation voltage, i.e. Ue y Ui.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Icu cos ϕ<br />
6 kA < Icu y 10 kA 0.5<br />
10 kA < Icu y 20 kA 0.3<br />
20 kA < Icu y 50 kA 0.25<br />
50 kA < Icu 0.2<br />
Fig. H34 : Icu related to power factor (cos ϕ) <strong>of</strong> fault-current circuit (IEC 60947-2)<br />
H15<br />
© Schneider Electric - all rights reserved
H16<br />
© Schneider Electric - all rights reserved<br />
H - LV switchgear: functions & selection<br />
t (s)<br />
Im<br />
Fig. H35 : Category A circuit-breaker<br />
t (s )<br />
Im<br />
Fig. H36 : Category B circuit-breaker<br />
In a correctly <strong>design</strong>ed <strong>installation</strong>, a circuitbreaker<br />
is never required to operate at its<br />
maximum breaking current Icu. For this reason<br />
a new characteristic Ics has been introduced.<br />
It is expressed in IEC 60947-2 as a percentage<br />
<strong>of</strong> Icu (25, 50, 75, 100%)<br />
(1) O represents an opening operation.<br />
CO represents a closing operation followed by an opening<br />
operation.<br />
I<br />
Icw<br />
Icu<br />
I(A)<br />
I(A )<br />
4 Circuit-breaker<br />
Rated impulse-withstand voltage (Uimp)<br />
This characteristic expresses, in kV peak (<strong>of</strong> a prescribed form and polarity) the value<br />
<strong>of</strong> voltage which the equipment is capable <strong>of</strong> withstanding without failure, under test<br />
conditions.<br />
<strong>General</strong>ly, for industrial circuit-breakers, Uimp = 8 kV and for domestic types,<br />
Uimp = 6 kV.<br />
Category (A or B) and rated short-time withstand current (Icw)<br />
As already briefly mentioned (sub-clause 4.2) there are two categories <strong>of</strong><br />
LV industrial switchgear, A and B, according to IEC 60947-2:<br />
b Those <strong>of</strong> category A, for which there is no deliberate delay in the operation <strong>of</strong> the<br />
“instantaneous” short-circuit magnetic tripping device (see Fig. H35), are generally<br />
moulded-case type circuit-breakers, and<br />
b Those <strong>of</strong> category B for which, in order to discriminate with other circuit-breakers<br />
on a time basis, it is possible to delay the tripping <strong>of</strong> the CB, where the fault-current<br />
level is lower than that <strong>of</strong> the short-time withstand current rating (Icw) <strong>of</strong> the CB<br />
(see Fig. H36). This is generally applied to large open-type circuit-breakers and<br />
to certain heavy-duty moulded-case types. Icw is the maximum current that the B<br />
category CB can withstand, thermally and electrodynamically, without sustaining<br />
damage, for a period <strong>of</strong> time given by the manufacturer.<br />
Rated making capacity (Icm)<br />
Icm is the highest instantaneous value <strong>of</strong> current that the circuit-breaker can<br />
establish at rated voltage in specified conditions. In AC systems this instantaneous<br />
peak value is related to Icu (i.e. to the rated breaking current) by the factor k, which<br />
depends on the power factor (cos ϕ) <strong>of</strong> the short-circuit current loop (as shown in<br />
Figure H37 ).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Icu cos ϕ Icm = kIcu<br />
6 kA < Icu y 10 kA 0.5 1.7 x Icu<br />
10 kA < Icu y 20 kA 0.3 2 x Icu<br />
20 kA < Icu y 50 kA 0.25 2.1 x Icu<br />
50 kA y Icu 0.2 2.2 x Icu<br />
Fig. H37 : Relation between rated breaking capacity Icu and rated making capacity Icm at<br />
different power-factor values <strong>of</strong> short-circuit current, as standardized in IEC 60947-2<br />
Example: A Masterpact NW08H2 circuit-breaker has a rated breaking capacity<br />
Icu <strong>of</strong> 100 kA. The peak value <strong>of</strong> its rated making capacity Icm will be<br />
100 x 2.2 = 220 kA.<br />
Rated service short-circuit breaking capacity (Ics)<br />
The rated breaking capacity (Icu) or (Icn) is the maximum fault-current a circuitbreaker<br />
can successfully interrupt without being damaged. The probability <strong>of</strong> such<br />
a current occurring is extremely low, and in normal circumstances the fault-currents<br />
are considerably less than the rated breaking capacity (Icu) <strong>of</strong> the CB. On the other<br />
hand it is important that high currents (<strong>of</strong> low probability) be interrupted under good<br />
conditions, so that the CB is immediately available for reclosure, after the faulty<br />
circuit has been repaired. It is for these reasons that a new characteristic (Ics) has<br />
been created, expressed as a percentage <strong>of</strong> Icu, viz: 25, 50, 75, 100% for industrial<br />
circuit-breakers. The standard test sequence is as follows:<br />
b O - CO - CO (1) (at Ics)<br />
b Tests carried out following this sequence are intended to verify that the CB is in a<br />
good state and available for normal service<br />
For domestic CBs, Ics = k Icn. The factor k values are given in IEC 60898 table XIV.<br />
In Europe it is the industrial practice to use a k factor <strong>of</strong> 100% so that Ics = Icu.
H - LV switchgear: functions & selection<br />
Many <strong>design</strong>s <strong>of</strong> LV circuit-breakers feature<br />
a short-circuit current limitation capability,<br />
whereby the current is reduced and prevented<br />
from reaching its (otherwise) maximum peak<br />
value (see Fig. H38). The current-limitation<br />
performance <strong>of</strong> these CBs is presented in<br />
the form <strong>of</strong> graphs, typified by that shown in<br />
Figure H39, diagram (a)<br />
Current limitation reduces both thermal and<br />
electrodynamic stresses on all circuit elements<br />
through which the current passes, thereby<br />
prolonging the useful life <strong>of</strong> these elements.<br />
Furthermore, the limitation feature allows<br />
“cascading” techniques to be used (see 4.5)<br />
thereby significantly reducing <strong>design</strong> and<br />
<strong>installation</strong> costs<br />
Icc<br />
Prospectice<br />
fault-current peak<br />
Limited<br />
current peak<br />
Limited<br />
current<br />
tc<br />
Fig. H38 : Prospective and actual currents<br />
Prospectice<br />
fault-current<br />
t<br />
4 Circuit-breaker<br />
Fault-current limitation<br />
The fault-current limitation capacity <strong>of</strong> a CB concerns its ability, more or less<br />
effective, in preventing the passage <strong>of</strong> the maximum prospective fault-current,<br />
permitting only a limited amount <strong>of</strong> current to flow, as shown in Figure H38.<br />
The current-limitation performance is given by the CB manufacturer in the form <strong>of</strong><br />
curves (see Fig. H39).<br />
b Diagram (a) shows the limited peak value <strong>of</strong> current plotted against the rms<br />
value <strong>of</strong> the AC component <strong>of</strong> the prospective fault current (“prospective” faultcurrent<br />
refers to the fault-current which would flow if the CB had no current-limiting<br />
capability)<br />
b Limitation <strong>of</strong> the current greatly reduces the thermal stresses (proportional I 2 t) and<br />
this is shown by the curve <strong>of</strong> diagram (b) <strong>of</strong> Figure H39, again, versus the rms value<br />
<strong>of</strong> the AC component <strong>of</strong> the prospective fault current.<br />
LV circuit-breakers for domestic and similar <strong>installation</strong>s are classified in certain<br />
standards (notably European Standard EN 60 898). CBs belonging to one class (<strong>of</strong><br />
current limiters) have standardized limiting I 2 t let-through characteristics defined by<br />
that class.<br />
In these cases, manufacturers do not normally provide characteristic performance<br />
curves.<br />
a)<br />
Limited<br />
current<br />
peak<br />
(kA)<br />
22<br />
Non-limited current<br />
characteristics<br />
Prospective AC<br />
component (rms)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
150 kA<br />
b)<br />
4,5.105<br />
2.105<br />
Limited<br />
current peak<br />
(A2 x s)<br />
Fig. H39 : Performance curves <strong>of</strong> a typical LV current-limiting circuit-breaker<br />
Prospective AC<br />
component (rms)<br />
150 kA<br />
The advantages <strong>of</strong> current limitation<br />
The use <strong>of</strong> current-limiting CBs affords numerous advantages:<br />
b Better conservation <strong>of</strong> <strong>installation</strong> networks: current-limiting CBs strongly attenuate<br />
all harmful effects associated with short-circuit currents<br />
b Reduction <strong>of</strong> thermal effects: Conductors (and therefore insulation) heating is<br />
significantly reduced, so that the life <strong>of</strong> cables is correspondingly increased<br />
b Reduction <strong>of</strong> mechanical effects: forces due to electromagnetic repulsion are lower,<br />
with less risk <strong>of</strong> deformation and possible rupture, excessive burning <strong>of</strong> contacts, etc.<br />
b Reduction <strong>of</strong> electromagnetic-interference effects:<br />
v Less influence on measuring instruments and associated circuits,<br />
telecommunication systems, etc.<br />
These circuit-breakers therefore contribute towards an improved exploitation <strong>of</strong>:<br />
b Cables and wiring<br />
b Prefabricated cable-trunking systems<br />
b Switchgear, thereby reducing the ageing <strong>of</strong> the <strong>installation</strong><br />
Example<br />
On a system having a prospective shortcircuit current <strong>of</strong> 150 kA rms, a Compact L<br />
circuit-breaker limits the peak current to less than 10% <strong>of</strong> the calculated prospective<br />
peak value, and the thermal effects to less than 1% <strong>of</strong> those calculated.<br />
Cascading <strong>of</strong> the several levels <strong>of</strong> distribution in an <strong>installation</strong>, downstream <strong>of</strong> a<br />
limiting CB, will also result in important savings.<br />
The technique <strong>of</strong> cascading, described in sub-clause 4.5 allows, in fact, substantial<br />
savings on switchgear (lower performance permissible downstream <strong>of</strong> the limiting<br />
CB(s)) enclosures, and <strong>design</strong> studies, <strong>of</strong> up to 20% (overall).<br />
Discriminative protection schemes and cascading are compatible, in the Compact<br />
NS range, up to the full short-circuit breaking capacity <strong>of</strong> the switchgear.<br />
H17<br />
© Schneider Electric - all rights reserved
H18<br />
© Schneider Electric - all rights reserved<br />
H - LV switchgear: functions & selection<br />
The choice <strong>of</strong> a range <strong>of</strong> circuit-breakers is<br />
determined by: the <strong>electrical</strong> characteristics <strong>of</strong><br />
the <strong>installation</strong>, the environment, the loads and<br />
a need for remote control, together with the type<br />
<strong>of</strong> telecommunications system envisaged<br />
Circuit-breakers with uncompensated thermal<br />
tripping units have a trip current level that<br />
depends on the surrounding temperature<br />
Ambient<br />
temperature<br />
Single CB<br />
in free air<br />
Fig. H40 : Ambient temperature<br />
Temperature <strong>of</strong> air<br />
surrouding the<br />
circuit breakers<br />
Circuit breakers installed<br />
in an enclosure<br />
Ambient<br />
temperature<br />
4 Circuit-breaker<br />
4.4 Selection <strong>of</strong> a circuit-breaker<br />
Choice <strong>of</strong> a circuit-breaker<br />
The choice <strong>of</strong> a CB is made in terms <strong>of</strong>:<br />
b Electrical characteristics <strong>of</strong> the <strong>installation</strong> for which the CB is intended<br />
b Its eventual environment: ambient temperature, in a kiosk or switchboard<br />
enclosure, climatic conditions, etc.<br />
b Short-circuit current breaking and making requirements<br />
b Operational specifications: discriminative tripping, requirements (or not) for<br />
remote control and indication and related auxiliary contacts, auxiliary tripping coils,<br />
connection<br />
b Installation regulations; in particular: protection <strong>of</strong> persons<br />
b Load characteristics, such as motors, fluorescent lighting, LV/LV transformers<br />
The following notes relate to the choice LV circuit-breaker for use in distribution<br />
systems.<br />
Choice <strong>of</strong> rated current in terms <strong>of</strong> ambient temperature<br />
The rated current <strong>of</strong> a circuit-breaker is defined for operation at a given ambient<br />
temperature, in general:<br />
b 30 °C for domestic-type CBs<br />
b 40 °C for industrial-type CBs<br />
Performance <strong>of</strong> these CBs in a different ambient temperature depends mainly on the<br />
technology <strong>of</strong> their tripping units (see Fig. H40).<br />
Uncompensated thermal magnetic tripping units<br />
Circuit-breakers with uncompensated thermal tripping elements have a trippingcurrent<br />
level that depends on the surrounding temperature. If the CB is installed<br />
in an enclosure, or in a hot location (boiler room, etc.), the current required to trip<br />
the CB on overload will be sensibly reduced. When the temperature in which the<br />
CB is located exceeds its reference temperature, it will therefore be “derated”. For<br />
this reason, CB manufacturers provide tables which indicate factors to apply at<br />
temperatures different to the CB reference temperature. It may be noted from typical<br />
examples <strong>of</strong> such tables (see Fig. H41) that a lower temperature than the reference<br />
value produces an up-rating <strong>of</strong> the CB. Moreover, small modular-type CBs mounted<br />
in juxtaposition, as shown typically in Figure H27, are usually mounted in a small<br />
closed metal case. In this situation, mutual heating, when passing normal load<br />
currents, generally requires them to be derated by a factor <strong>of</strong> 0.8.<br />
C60a, C60H: curve C. C60N: curves B and C (reference temperature: 30 °C)<br />
Rating (A) 20 °C 25 °C 30 °C 35 °C 40 °C 45 °C 50 °C 55 °C 60 °C<br />
1 1.05 1.02 1.00 0.98 0.95 0.93 0.90 0.88 0.85<br />
2 2.08 2.04 2.00 1.96 1.92 1.88 1.84 1.80 1.74<br />
3 3.18 3.09 3.00 2.91 2.82 2.70 2.61 2.49 2.37<br />
4 4.24 4.12 4.00 3.88 3.76 3.64 3.52 3.36 3.24<br />
6 6.24 6.12 6.00 5.88 5.76 5.64 5.52 5.40 5.30<br />
10 10.6 10.3 10.0 9.70 9.30 9.00 8.60 8.20 7.80<br />
16 16.8 16.5 16.0 15.5 15.2 14.7 14.2 13.8 13.5<br />
20 21.0 20.6 20.0 19.4 19.0 18.4 17.8 17.4 16.8<br />
25 26.2 25.7 25.0 24.2 23.7 23.0 22.2 21.5 20.7<br />
32 33.5 32.9 32.0 31.4 30.4 29.8 28.4 28.2 27.5<br />
40 42.0 41.2 40.0 38.8 38.0 36.8 35.6 34.4 33.2<br />
50 52.5 51.5 50.0 48.5 47.4 45.5 44.0 42.5 40.5<br />
63 66.2 64.9 63.0 61.1 58.0 56.7 54.2 51.7 49.2<br />
NS250N/H/L (reference temperature: 40 °C)<br />
Rating (A) 40 °C 45 °C 50 °C 55 °C 60 °C<br />
TM160D 160 156 152 147 144<br />
TM200D 200 195 190 185 180<br />
TM250D 250 244 238 231 225<br />
Fig. H41 : Examples <strong>of</strong> tables for the determination <strong>of</strong> derating/uprating factors to apply to CBs<br />
with uncompensated thermal tripping units, according to temperature<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
H - LV switchgear: functions & selection<br />
Electronic tripping units are highly stable in<br />
changing temperature levels<br />
4 Circuit-breaker<br />
Example<br />
What rating (In) should be selected for a C60 N?<br />
b Protecting a circuit, the maximum load current <strong>of</strong> which is estimated to be 34 A<br />
b Installed side-by-side with other CBs in a closed distribution box<br />
b In an ambient temperature <strong>of</strong> 50 °C<br />
A C60N circuit-breaker rated at 40 A would be derated to 35.6 A in ambient air at<br />
50 °C (see Fig. H41). To allow for mutual heating in the enclosed space, however, the<br />
0.8 factor noted above must be employed, so that, 35.6 x 0.8 = 28.5 A, which is not<br />
suitable for the 34 A load.<br />
A 50 A circuit-breaker would therefore be selected, giving a (derated) current rating<br />
<strong>of</strong> 44 x 0.8 = 35.2 A.<br />
Compensated thermal-magnetic tripping units<br />
These tripping units include a bi-metal compensating strip which allows the overload<br />
trip-current setting (Ir or Irth) to be adjusted, within a specified range, irrespective <strong>of</strong><br />
the ambient temperature.<br />
For example:<br />
b In certain countries, the TT system is standard on LV distribution systems, and<br />
domestic (and similar) <strong>installation</strong>s are protected at the service position by a circuitbreaker<br />
provided by the supply authority. This CB, besides affording protection<br />
against indirect-contact hazard, will trip on overload; in this case, if the consumer<br />
exceeds the current level stated in his supply contract with the power authority. The<br />
circuit-breaker (y 60 A) is compensated for a temperature range <strong>of</strong> - 5 °C to + 40 °C.<br />
b LV circuit-breakers at ratings y 630 A are commonly equipped with compensated<br />
tripping units for this range (- 5 °C to + 40 °C)<br />
Electronic tripping units<br />
An important advantage with electronic tripping units is their stable performance<br />
in changing temperature conditions. However, the switchgear itself <strong>of</strong>ten imposes<br />
operational limits in elevated temperatures, so that manufacturers generally provide<br />
an operating chart relating the maximum values <strong>of</strong> permissible trip-current levels to<br />
the ambient temperature (see Fig. H42).<br />
Masterpact NW20 version 40°C 45°C 50°C 55°C 60°C<br />
H1/H2/H3 Withdrawable with In (A) 2,000 2,000 2,000 1,980 1,890<br />
horizontal plugs Maximum 1 1 1 0.99 0.95<br />
adjustment Ir<br />
L1 Withdrawable with In (A) 2,000 200 1,900 1,850 1,800<br />
on-edge plugs Maximum 1 1 0.95 0.93 0.90<br />
adjustment Ir<br />
Coeff.<br />
1 2,000<br />
0.95 1,890<br />
0.90 1,800<br />
In (A)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
NW20 withdrawable with<br />
horizontal plugs<br />
NW20 L1 withdrawable<br />
with on edge plugs<br />
20 25 30 35 40 45 50 55 60<br />
Fig. H42 : Derating <strong>of</strong> Masterpact NW20 circuit-breaker, according to the temperature<br />
θ°C<br />
H19<br />
© Schneider Electric - all rights reserved
H20<br />
© Schneider Electric - all rights reserved<br />
H - LV switchgear: functions & selection<br />
The <strong>installation</strong> <strong>of</strong> a LV circuit-breaker requires<br />
that its short-circuit breaking capacity (or that <strong>of</strong><br />
the CB together with an associated device) be<br />
equal to or exceeds the calculated prospective<br />
short-circuit current at its point <strong>of</strong> <strong>installation</strong><br />
The circuit-breaker at the output <strong>of</strong> the smallest<br />
transformer must have a short-circuit capacity<br />
adequate for a fault current which is higher<br />
than that through any <strong>of</strong> the other transformer<br />
LV circuit-breakers<br />
4 Circuit-breaker<br />
Selection <strong>of</strong> an instantaneous, or short-time-delay, tripping<br />
threshold<br />
Figure H43 below summarizes the main characteristics <strong>of</strong> the instantaneous or<br />
short-time delay trip units.<br />
Type Tripping unit Applications<br />
t<br />
Low setting<br />
type B<br />
b Sources producing low short-circuitcurrent<br />
levels<br />
(standby generators)<br />
b Long lengths <strong>of</strong> line or cable<br />
t<br />
t<br />
t<br />
Fig. H43 : Different tripping units, instantaneous or short-time-delayed<br />
Selection <strong>of</strong> a circuit-breaker according to the short-circuit<br />
breaking capacity requirements<br />
The <strong>installation</strong> <strong>of</strong> a circuit-breaker in a LV <strong>installation</strong> must fulfil one <strong>of</strong> the two<br />
following conditions:<br />
b Either have a rated short-circuit breaking capacity Icu (or Icn) which is equal to or<br />
exceeds the prospective short-circuit current calculated for its point <strong>of</strong> <strong>installation</strong>, or<br />
b If this is not the case, be associated with another device which is located<br />
upstream, and which has the required short-circuit breaking capacity<br />
In the second case, the characteristics <strong>of</strong> the two devices must be co-ordinated<br />
such that the energy permitted to pass through the upstream device must not<br />
exceed that which the downstream device and all associated cables, wires and other<br />
components can withstand, without being damaged in any way. This technique is<br />
pr<strong>of</strong>itably employed in:<br />
b Associations <strong>of</strong> fuses and circuit-breakers<br />
b Associations <strong>of</strong> current-limiting circuit-breakers and standard circuit-breakers.<br />
The technique is known as “cascading” (see sub-clause 4.5 <strong>of</strong> this chapter)<br />
The selection <strong>of</strong> main and principal circuit-breakers<br />
A single transformer<br />
If the transformer is located in a consumer’s substation, certain national standards<br />
require a LV circuit-breaker in which the open contacts are clearly visible such as<br />
Compact NS withdrawable circuit-breaker.<br />
Example (see Fig. H44 opposite page)<br />
What type <strong>of</strong> circuit-breaker is suitable for the main circuit-breaker <strong>of</strong> an <strong>installation</strong><br />
supplied through a 250 kVA MV/LV (400 V) 3-phase transformer in a consumer’s<br />
substation?<br />
In transformer = 360 A<br />
Isc (3-phase) = 8.9 kA<br />
A Compact NS400N with an adjustable tripping-unit range <strong>of</strong> 160 A - 400 A and a<br />
short-circuit breaking capacity (Icu) <strong>of</strong> 45 kA would be a suitable choice for this duty.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
I<br />
I<br />
I<br />
I<br />
Standard setting b Protection <strong>of</strong> circuits: general case<br />
type C<br />
High setting b Protection <strong>of</strong> circuits having high initial<br />
type D or K transient current levels<br />
(e.g. motors, transformers, resistive loads)<br />
12 In b Protection <strong>of</strong> motors in association with<br />
type MA discontactors<br />
(contactors with overload protection)
H - LV switchgear: functions & selection<br />
250 kVA<br />
20 kV/400 V<br />
Compact<br />
NS400N<br />
Fig. H44 : Example <strong>of</strong> a transformer in a consumer’s substation<br />
A1<br />
B1<br />
MV<br />
LV<br />
Tr1<br />
CBM<br />
CBP<br />
A2<br />
B2<br />
Fig. H45 : Transformers in parallel<br />
E<br />
MV<br />
LV<br />
Tr2<br />
CBM<br />
CBP<br />
A3<br />
B3<br />
MV<br />
LV<br />
Tr3<br />
CBM<br />
4 Circuit-breaker<br />
Several transformers in parallel (see Fig. H45)<br />
b The circuit-breakers CBP outgoing from the LV distribution board must each be<br />
capable <strong>of</strong> breaking the total fault current from all transformers connected to the<br />
busbars, viz: Isc1 + Isc2 + Isc3<br />
b The circuit-breakers CBM, each controlling the output <strong>of</strong> a transformer, must be<br />
capable <strong>of</strong> dealing with a maximum short-circuit current <strong>of</strong> (for example) Isc2 + Isc3<br />
only, for a short-circuit located on the upstream side <strong>of</strong> CBM1.<br />
From these considerations, it will be seen that the circuit-breaker <strong>of</strong> the smallest<br />
transformer will be subjected to the highest level <strong>of</strong> fault current in these<br />
circumstances, while the circuit-breaker <strong>of</strong> the largest transformer will pass the<br />
lowest level <strong>of</strong> short-circuit current<br />
b The ratings <strong>of</strong> CBMs must be chosen according to the kVA ratings <strong>of</strong> the<br />
associated transformers<br />
Note: The essential conditions for the successful operation <strong>of</strong> 3-phase transformers<br />
in parallel may be summarized as follows:<br />
1. the phase shift <strong>of</strong> the voltages, primary to secondary, must be the same in all units<br />
to be paralleled.<br />
2. the open-circuit voltage ratios, primary to secondary, must be the same in all units.<br />
3. the short-circuit impedance voltage (Zsc%) must be the same for all units.<br />
For example, a 750 kVA transformer with a Zsc = 6% will share the load correctly<br />
with a 1,000 kVA transformer having a Zsc <strong>of</strong> 6%, i.e. the transformers will be loaded<br />
automatically in proportion to their kVA ratings. For transformers having a ratio <strong>of</strong> kVA<br />
ratings exceeding 2, parallel operation is not recommended.<br />
Figure H46 indicates, for the most usual arrangement (2 or 3 transformers <strong>of</strong><br />
equal kVA ratings) the maximum short-circuit currents to which main and principal<br />
CBs (CBM and CBP respectively, in Figure H45) are subjected. It is based on the<br />
following hypotheses:<br />
b The short-circuit 3-phase power on the MV side <strong>of</strong> the transformer is 500 MVA<br />
b The transformers are standard 20/0.4 kV distribution-type units rated as listed<br />
b The cables from each transformer to its LV circuit-breaker comprise 5 metres <strong>of</strong><br />
single core conductors<br />
b Between each incoming-circuit CBM and each outgoing-circuit CBP there is<br />
1 metre <strong>of</strong> busbar<br />
b The switchgear is installed in a floormounted enclosed switchboard, in an ambientair<br />
temperature <strong>of</strong> 30 °C<br />
Moreover, this table shows selected circuit-breakers <strong>of</strong> M-G manufacture<br />
recommended for main and principal circuit-breakers in each case.<br />
Example (see Fig. H47 next page)<br />
b Circuit-breaker selection for CBM duty:<br />
For a 800 kVA transformer In = 1.126 A; Icu (minimum) = 38 kA (from Figure H46),<br />
the CBM indicated in the table is a Compact NS1250N (Icu = 50 kA)<br />
b Circuit-breaker selection for CBP duty:<br />
The s.c. breaking capacity (Icu) required for these circuit-breakers is given in the<br />
Figure H46 as 56 kA.<br />
A recommended choice for the three outgoing circuits 1, 2 and 3 would be currentlimiting<br />
circuit-breakers types NS400 L, NS250 L and NS100 L. The Icu rating in<br />
each case = 150 kA.<br />
Number and kVA ratings Minimum S.C breaking Main circuit-breakers (CBM) Minimum S.C breaking Rated current In <strong>of</strong><br />
<strong>of</strong> 20/0.4 kV transformers capacity <strong>of</strong> main CBs total discrimination with out capacity <strong>of</strong> principal CBs principal circuit-breaker<br />
(Icu) kA going circuit-breakers (CBP) (Icu) kA (CPB) 250A<br />
2 x 400 14 NW08N1/NS800N 27 NS250H<br />
3 x 400 28 NW08N1/NS800N 42 NS250H<br />
2 x 630 22 NW10N1/NS1000N 42 NS250H<br />
3 x 630 44 NW10N1/NS1000N 67 NS250H<br />
2 x 800 19 NW12N1/NS1250N 38 NS250H<br />
3 x 800 38 NW12N1/NS1250N 56 NS250H<br />
2 x 1,000 23 NW16N1/NS1600N 47 NS250H<br />
3 x 1,000 47 NW16N1/NS1600N 70 NS250H<br />
2 x 1,250 29 NW20N1/NS2000N 59 NS250H<br />
3 x 1,250 59 NW20N1/NS2000N 88 NS250L<br />
2 x 1,600 38 NW25N1/NS2500N 75 NS250L<br />
3 x 1,600 75 NW25N1/NS2500N 113 NS250L<br />
2 x 2,000 47 NW32N1/NS3200N 94 NS250L<br />
3 x 2,000 94 NW32N1/NS3200N 141 NS250L<br />
Fig. H46 : Maximum values <strong>of</strong> short-circuit current to be interrupted by main and principal circuit-breakers (CBM and CBP respectively), for several transformers in<br />
parallel<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
H21<br />
© Schneider Electric - all rights reserved
H22<br />
© Schneider Electric - all rights reserved<br />
H - LV switchgear: functions & selection<br />
Short-circuit fault-current levels at any point in<br />
an <strong>installation</strong> may be obtained from tables<br />
400 A<br />
CBP1<br />
100 A<br />
Fig. H47 : Transformers in parallel<br />
CBP2<br />
3 Tr<br />
800 kVA<br />
20 kV/400 V<br />
CBM<br />
200 A<br />
CBP3<br />
The technique <strong>of</strong> “cascading” uses the<br />
properties <strong>of</strong> current-limiting circuit-breakers<br />
to permit the <strong>installation</strong> <strong>of</strong> all downstream<br />
switchgear, cables and other circuit components<br />
<strong>of</strong> significantly lower performance than would<br />
otherwise be necessary, thereby simplifying and<br />
reducing the cost <strong>of</strong> an <strong>installation</strong><br />
4 Circuit-breaker<br />
These circuit-breakers provide the advantages <strong>of</strong>:<br />
v Absolute discrimination with the upstream (CBM) breakers<br />
v Exploitation <strong>of</strong> the “cascading” technique, with its associated savings for all<br />
downstream components<br />
Choice <strong>of</strong> outgoing-circuit CBs and final-circuit CBs<br />
Use <strong>of</strong> table G40<br />
From this table, the value <strong>of</strong> 3-phase short-circuit current can be determined rapidly<br />
for any point in the <strong>installation</strong>, knowing:<br />
b The value <strong>of</strong> short-circuit current at a point upstream <strong>of</strong> that intended for the CB<br />
concerned<br />
b The length, c.s.a., and the composition <strong>of</strong> the conductors between the two points<br />
A circuit-breaker rated for a short-circuit breaking capacity exceeding the tabulated<br />
value may then be selected.<br />
Detailed calculation <strong>of</strong> the short-circuit current level<br />
In order to calculate more precisely the short-circuit current, notably, when the shortcircuit<br />
current-breaking capacity <strong>of</strong> a CB is slightly less than that derived from the<br />
table, it is necessary to use the method indicated in chapter G clause 4.<br />
Two-pole circuit-breakers (for phase and neutral) with one protected pole only<br />
These CBs are generally provided with an overcurrent protective device on the<br />
phase pole only, and may be used in TT, TN-S and IT schemes. In an IT scheme,<br />
however, the following conditions must be respected:<br />
b Condition (B) <strong>of</strong> table G67 for the protection <strong>of</strong> the neutral conductor against<br />
overcurrent in the case <strong>of</strong> a double fault<br />
b Short-circuit current-breaking rating: A 2-pole phase-neutral CB must, by<br />
convention, be capable <strong>of</strong> breaking on one pole (at the phase-to-phase voltage) the<br />
current <strong>of</strong> a double fault equal to 15% <strong>of</strong> the 3-phase short-circuit current at the point<br />
<strong>of</strong> its <strong>installation</strong>, if that current is y 10 kA; or 25% <strong>of</strong> the 3-phase short-circuit current<br />
if it exceeds 10 kA<br />
b Protection against indirect contact: this protection is provided according to the<br />
<strong>rules</strong> for IT schemes<br />
Insufficient short-circuit current breaking rating<br />
In low-voltage distribution systems it sometimes happens, especially in heavy-duty<br />
networks, that the Isc calculated exceeds the Icu rating <strong>of</strong> the CBs available for<br />
<strong>installation</strong>, or system changes upstream result in lower level CB ratings being<br />
exceeded<br />
b Solution 1: Check whether or not appropriate CBs upstream <strong>of</strong> the CBs affected<br />
are <strong>of</strong> the current-limiting type, allowing the principle <strong>of</strong> cascading (described in subclause<br />
4.5) to be applied<br />
b Solution 2: Install a range <strong>of</strong> CBs having a higher rating. This solution is<br />
economically interesting only where one or two CBs are affected<br />
b Solution 3: Associate current-limiting fuses (gG or aM) with the CBs concerned, on<br />
the upstream side. This arrangement must, however, respect the following <strong>rules</strong>:<br />
v The fuse rating must be appropriate<br />
v No fuse in the neutral conductor, except in certain IT <strong>installation</strong>s where a double<br />
fault produces a current in the neutral which exceeds the short-circuit breaking rating<br />
<strong>of</strong> the CB. In this case, the blowing <strong>of</strong> the neutral fuse must cause the CB to trip on<br />
all phases<br />
4.5 Coordination between circuit-breakers<br />
Cascading<br />
Definition <strong>of</strong> the cascading technique<br />
By limiting the peak value <strong>of</strong> short-circuit current passing through it, a currentlimiting<br />
CB permits the use, in all circuits downstream <strong>of</strong> its location, <strong>of</strong> switchgear<br />
and circuit components having much lower short-circuit breaking capacities, and<br />
thermal and electromechanical withstand capabilities than would otherwise be<br />
necessary. Reduced physical size and lower performance requirements lead to<br />
substantial economy and to the simplification <strong>of</strong> <strong>installation</strong> work. It may be noted<br />
that, while a current-limiting circuit-breaker has the effect on downstream circuits<br />
<strong>of</strong> (apparently) increasing the source impedance during short-circuit conditions, it<br />
has no such effect in any other condition; for example, during the starting <strong>of</strong> a large<br />
motor (where a low source impedance is highly desirable). The range <strong>of</strong> Compact<br />
NS current-limiting circuit-breakers with powerful limiting performances is particularly<br />
interesting.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
H - LV switchgear: functions & selection<br />
In general, laboratory tests are necessary to<br />
ensure that the conditions <strong>of</strong> implementation<br />
required by national standards are met and<br />
compatible switchgear combinations must be<br />
provided by the manufacturer<br />
Discrimination may be total or partial, and<br />
based on the principles <strong>of</strong> current levels, or<br />
time-delays, or a combination <strong>of</strong> both. A more<br />
recent development is based on the logic<br />
techniques.<br />
A (patented) system by Merlin Gerin takes<br />
advantages <strong>of</strong> both current-limitation and<br />
discrimination<br />
4 Circuit-breaker<br />
Conditions <strong>of</strong> implementation<br />
Most national standards admit the cascading technique, on condition that the<br />
amount <strong>of</strong> energy “let through” by the limiting CB is less than the energy all<br />
downstream CBs and components are able to withstand without damage.<br />
In practice this can only be verified for CBs by tests performed in a laboratory. Such<br />
tests are carried out by manufacturers who provide the information in the form <strong>of</strong><br />
tables, so that users can confidently <strong>design</strong> a cascading scheme based on the<br />
combination <strong>of</strong> recommended circuit-breaker types. As an example, Figure H48<br />
indicates the cascading possibilities <strong>of</strong> circuit-breaker types C60, DT40N, C120 and<br />
NG125 when installed downstream <strong>of</strong> current-limiting CBs NS 250 N, H or L for a<br />
230/400 V or 240/415 V 3-phase <strong>installation</strong>.<br />
kA rms<br />
Short-circuit 150 NS250L<br />
breaking capacity 50 NS250H<br />
<strong>of</strong> the upstream 35 NS250N<br />
(limiter) CBs<br />
Possible short-circuit 150 NG125L<br />
breaking capacity <strong>of</strong> 70 NG125L<br />
the downstream CBs 40 C60L y 40 A C60L y 40 A<br />
(benefiting from the 36 NG125N NG125N<br />
cascading technique) 30 C60H C60N/H/L C60N/H<br />
C60L C60L 50-63 A<br />
25 C60N C120N/H<br />
C120N/H C120N/H<br />
20 DT40N DT40N DT40N<br />
Fig. H48 : Example <strong>of</strong> cascading possibilities on a 230/400 V or 240/415 V 3-phase <strong>installation</strong><br />
Advantages <strong>of</strong> cascading<br />
The current limitation benefits all downstream circuits that are controlled by the<br />
current-limiting CB concerned.<br />
The principle is not restrictive, i.e. current-limiting CBs can be installed at any point in<br />
an <strong>installation</strong> where the downstream circuits would otherwise be inadequately rated.<br />
The result is:<br />
b Simplified short-circuit current calculations<br />
b Simplification, i.e. a wider choice <strong>of</strong> downstream switchgear and appliances<br />
b The use <strong>of</strong> lighter-duty switchgear and appliances, with consequently lower cost<br />
b Economy <strong>of</strong> space requirements, since light-duty equipment have generally a<br />
smaller volume<br />
Principles <strong>of</strong> discriminative tripping (selectivity)<br />
Discrimination is achieved by automatic protective devices if a fault condition, occurring<br />
at any point in the <strong>installation</strong>, is cleared by the protective device located immediately<br />
upstream <strong>of</strong> the fault, while all other protective devices remain unaffected (see<br />
Fig. H49).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
B<br />
Isc<br />
0<br />
Total discrimination<br />
Ir B<br />
Partial discrimination<br />
Isc B<br />
Isc<br />
0<br />
B only opens A and B open<br />
Ir B Is Isc B<br />
Isc<br />
Fig. H49 : Total and partial discrimination<br />
A<br />
Is = discrimination limit<br />
H23<br />
© Schneider Electric - all rights reserved
H24<br />
© Schneider Electric - all rights reserved<br />
H - LV switchgear: functions & selection<br />
t<br />
t<br />
Ir B<br />
Ir B<br />
Ir A<br />
Ir A<br />
B A<br />
B A<br />
B only opens<br />
Isc B Im A<br />
Fig. H50 : Total discrimination between CBs A and B<br />
Im A Is c B<br />
A and B open<br />
Is c A<br />
Fig. H51 : Partial discrimination between CBs A and B<br />
a)<br />
b)<br />
c)<br />
t<br />
t<br />
t<br />
B<br />
Ir B<br />
B A<br />
A<br />
Fig. H52 : Discrimination<br />
Ir A<br />
B A<br />
Im A<br />
delayed<br />
B<br />
A<br />
Isc B<br />
Isc B<br />
∆t<br />
Ii A<br />
instantaneous<br />
I<br />
I<br />
I<br />
I<br />
I<br />
4 Circuit-breaker<br />
Discrimination between circuit-breakers A and B is total if the maximum value <strong>of</strong><br />
short-circuit-current on circuit B (Isc B) does not exceed the short-circuit trip setting<br />
<strong>of</strong> circuit-breaker A (Im A). For this condition, B only will trip (see Fig. H50).<br />
Discrimination is partial if the maximum possible short-circuit current on circuit B<br />
exceeds the short-circuit trip-current setting <strong>of</strong> circuit-breaker A. For this maximum<br />
condition, both A and B will trip (see Fig. H51).<br />
Protection against overload : discrimination based on current levels<br />
(see Fig. H52a)<br />
This method is realized by setting successive tripping thresholds at stepped levels,<br />
from downstream relays (lower settings) towards the source (higher settings).<br />
Discrimination is total or partial, depending on particular conditions, as noted above.<br />
As a rule <strong>of</strong> thumb, discrimination is achieved when:<br />
b IrA/IrB > 2:<br />
Protection against low level short-circuit currents : discrimination based on<br />
stepped time delays (see Fig. H52b)<br />
This method is implemented by adjusting the time-delayed tripping units, such that<br />
downstream relays have the shortest operating times, with progressively longer<br />
delays towards the source.<br />
In the two-level arrangement shown, upstream circuit-breaker A is delayed<br />
sufficiently to ensure total discrimination with B (for example: Masterpact with<br />
electronic trip unit).<br />
Discrimination based on a combination <strong>of</strong> the two previous methods<br />
(see Fig. H52c)<br />
A time-delay added to a current level scheme can improve the overall discrimination<br />
performance.<br />
The upstream CB has two high-speed magnetic tripping thresholds:<br />
b Im A: delayed magnetic trip or short-delay electronic trip<br />
b Ii: instantaneous strip<br />
Discrimination is total if Isc B < Ii (instantaneous).<br />
Protection against high level short-circuit currents: discrimination based on<br />
arc-energy levels<br />
This technology implemented in the Compact NS range (current limiting circuit-<br />
breaker) is extremely effective for achievement <strong>of</strong> total discrimination.<br />
Principle: When a very high level short-circuit current is detected by the two circuits-<br />
breaker A and B, their contacts open simultaneously. As a result, the current is highly<br />
limited.<br />
b The very high arc-energy at level B induces the tripping <strong>of</strong> circuit-breaker B<br />
b Then, the arc-energy is limited at level A and is not sufficient to induce the tripping<br />
<strong>of</strong> A<br />
As a rule <strong>of</strong> thumb, the discrimination between Compact NS is total if the size ratio<br />
between A and B is greater than 2.5.<br />
Current-level discrimination<br />
Current-level discrimination is achieved with stepped current-level settings<br />
<strong>of</strong> the instantaneous magnetic-trip elements<br />
Current-level discrimination is achieved with circuits breakers, preferably currentlimiting,<br />
and stepped current-level settings <strong>of</strong> the instantaneous magnetic-trip<br />
elements.<br />
b The downstream circuit-breaker is not a current-limiter device<br />
Total discrimination in this situation is practically impossible because<br />
Isc A ≈ Isc B, so that both circuit-breakers will generally trip simultaneously. In this<br />
case discrimination is partial, and limited to the Im <strong>of</strong> the upstream circuit-breaker.<br />
See fig. H51.<br />
b The downstream circuit-breaker is a current-limiting device<br />
Improvement in discriminative tripping can be obtained by using a current limiter<br />
for circuit-breaker B. For a short-circuit downstream <strong>of</strong> B, the limited level <strong>of</strong> peak<br />
current IB would operate the (suitably adjusted) magnetic trip unit <strong>of</strong> B, but would be<br />
insufficient to cause circuit-breaker A to trip.<br />
Note: All LV breakers (considered here) have some inherent degree <strong>of</strong> current<br />
limitation, even those that are not classified as current-limiting. This accounts for<br />
the curved characteristic shown for the standard circuit-breaker A in Figure H53<br />
opposite page. Careful calculation and testing is necessary, however, to ensure<br />
satisfactory performance <strong>of</strong> this arrangement.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
H - LV switchgear: functions & selection<br />
Discrimination based on time-delayed tripping<br />
uses CBs referred to as “selective” (in some<br />
countries).<br />
Implementation <strong>of</strong> these CBs is relatively simple<br />
and consists in delaying the instant <strong>of</strong> tripping<br />
<strong>of</strong> the several series-connected circuit-breakers<br />
in a stepped time sequence<br />
(1) Short-delay<br />
4 Circuit-breaker<br />
Fault<br />
upstream<br />
<strong>of</strong> B<br />
Fault<br />
downstream<br />
<strong>of</strong> B<br />
b The upstream circuit-breaker is a high speed device with a short-delay (SD) feature<br />
These circuit-breakers are fitted with trip units which include a non-adjustable<br />
mechanical short-time-delay feature. The delay is sufficient to ensure total<br />
discrimination with any downstream high-speed CB at any value <strong>of</strong> short-circuit<br />
current up to Ii A (see Fig. H54).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
I peak<br />
Fig. H53 : Downstream limiting circuit-breaker B<br />
t<br />
A<br />
Is c<br />
Fig. H54 : Use <strong>of</strong> a “selective” circuit-breaker upstream<br />
B<br />
A<br />
Current limitation<br />
curves<br />
(see note)<br />
B<br />
Only B opens A and B open<br />
Im A<br />
delayed<br />
Is c<br />
prospective (rms)<br />
Ii A<br />
instantaneous<br />
Example<br />
Circuit-breaker A: Compact NS250 N fitted with a trip unit which includes a SD<br />
feature.<br />
Ir = 250 A, magnetic trip set at 2,000 A<br />
Circuit-breaker B: Compact NS100N<br />
Ir = 100 A<br />
The Merlin Gerin distribution catalogue indicates a discrimination limit <strong>of</strong> 3,000 A (an<br />
improvement over the limit <strong>of</strong> 2,500 A obtained when using a standard tripping unit).<br />
Time-based discrimination<br />
This technique requires:<br />
b The introduction <strong>of</strong> time-delays into the tripping mechanisms <strong>of</strong> CBs<br />
b CBs with adequate thermal and mechanical withstand capabilities at the high<br />
current levels and time delays considered<br />
Two circuit-breakers A and B in series (i.e. carrying the same current) are<br />
discriminative if the current-breaking period <strong>of</strong> downstream circuit-breaker B is less<br />
than the non-tripping time <strong>of</strong> circuit-breaker A.<br />
I<br />
H25<br />
© Schneider Electric - all rights reserved
H26<br />
© Schneider Electric - all rights reserved<br />
H - LV switchgear: functions & selection<br />
4 Circuit-breaker<br />
Practical example <strong>of</strong> discrimination at several levels with Merlin Gerin<br />
Masterpact circuit-breakers (with electronic trip units)<br />
These CBs can be equipped with adjustable time-delays which allow 4 time-step<br />
selections, such as:<br />
b The delay corresponding to a given step is greater than the total current breaking<br />
time <strong>of</strong> the next lower step<br />
b The delay corresponding to the first step is greater than the total current-breaking<br />
time <strong>of</strong> a high-speed CB (Compact NS for example) or <strong>of</strong> fuses (see Fig. H55)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
t<br />
Ir B<br />
Fig. H55 : Discrimination by time delay<br />
B<br />
A<br />
Only B opens<br />
Non tripping<br />
time <strong>of</strong> A<br />
Current-breaking<br />
time for B<br />
Icc B<br />
Energy discrimination with current limitation<br />
I<br />
Icc<br />
Cascading between 2 devices is normally achieved by using the tripping <strong>of</strong> the<br />
upstream circuit-breaker A to help the downstream circuit-breaker B to break the<br />
current. The discrimination limit Is is consequently equal to the ultimate breaking<br />
current Icu B <strong>of</strong> circuit-breaker B acting alone, as cascading requires the tripping <strong>of</strong><br />
both devices.<br />
The energy discrimination technology implemented in Compact NS circuit-breakers<br />
allows to improve the discrimination limit to a value higher than the ultimate breaking<br />
current Icu B <strong>of</strong> the downstream circuit-breaker. The principle is as follows:<br />
b The downstream limiting circuit-breaker B sees a very high short-circuit current.<br />
The tripping is very fast ( 1.6<br />
b The ratio <strong>of</strong> rated currents <strong>of</strong> the two circuit-breakers is > 2.5
H - LV switchgear: functions & selection<br />
Discrimination schemes based on logic<br />
techniques are possible, using CBs equipped<br />
with electronic tripping units <strong>design</strong>ed for the<br />
purpose (Compact, Masterpact by MG) and<br />
interconnected with pilot wires<br />
4 Circuit-breaker<br />
Logic discrimination or “Zone Sequence Interlocking – ZSI”<br />
This discrimination system requires CBs equipped with electronic tripping units,<br />
<strong>design</strong>ed for this application, together with interconnecting pilot wires for data<br />
exchange between the CBs. With 2 levels A and B (see Fig. H56), circuit-breaker A<br />
is set to trip instantaneously, unless the relay <strong>of</strong> circuit-breaker B sends a signal to<br />
confirm that the fault is downstream <strong>of</strong> B. This signal causes the tripping unit <strong>of</strong> A to<br />
be delayed, thereby ensuring back-up protection in the event that B fails to clear the<br />
fault, and so on…<br />
This system (patented by Merlin Gerin) also allows rapid localization <strong>of</strong> the fault.<br />
Fig. H56 : Logic discrimination<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
A<br />
B<br />
Inter-locking<br />
order<br />
Pilot wire<br />
4.6 Discrimination MV/LV in a consumer’s<br />
substation<br />
In general the transformer in a consumer’s substation is protected by MV fuses,<br />
suitably rated to match the transformer, in accordance with the principles laid down<br />
in IEC 60787 and IEC 60420, by following the advice <strong>of</strong> the fuse manufacturer.<br />
The basic requirement is that a MV fuse will not operate for LV faults occurring<br />
downstream <strong>of</strong> the transformer LV circuit-breaker, so that the tripping characteristic<br />
curve <strong>of</strong> the latter must be to the left <strong>of</strong> that <strong>of</strong> the MV fuse pre-arcing curve.<br />
This requirement generally fixes the maximum settings for the LV circuit-breaker<br />
protection:<br />
b Maximum short-circuit current-level setting <strong>of</strong> the magnetic tripping element<br />
b Maximum time-delay allowable for the short-circuit current tripping element<br />
(see Fig. H57)<br />
Fig. H57 : Example<br />
Full-load current<br />
1,760 A<br />
3-phase<br />
short-circuit<br />
current level<br />
31.4 kA<br />
63 A<br />
1,250 kVA<br />
20 kV / 400 V<br />
Compact<br />
NS2000<br />
set at 1,800 A<br />
H27<br />
© Schneider Electric - all rights reserved
H28<br />
© Schneider Electric - all rights reserved<br />
H - LV switchgear: functions & selection<br />
4 Circuit-breaker<br />
b Short-circuit level at MV terminals <strong>of</strong> transformer: 250 MVA<br />
b Transformer MV/LV: 1,250 kVA 20/0.4 kV<br />
b MV fuses: 63 A<br />
b Cabling, transformer - LV circuit-breaker: 10 metres single-core cables<br />
b LV circuit-breaker: Compact NS 2000 set at 1,800 A (Ir)<br />
What is the maximum short-circuit trip current setting and its maximum time delay<br />
allowable?<br />
The curves <strong>of</strong> Figure H58 show that discrimination is assured if the short-time delay<br />
tripping unit <strong>of</strong> the CB is set at:<br />
b A level y 6 Ir = 10.8 kA<br />
b A time-delay setting <strong>of</strong> step 1 or 2<br />
1,000<br />
200<br />
100<br />
10<br />
0.2<br />
0.1<br />
0.50<br />
0.01<br />
t<br />
(s)<br />
1,800 A<br />
Ir<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
NS 2000<br />
set at<br />
1,800 A<br />
1 4 6<br />
10 kA<br />
Fig. H58 : Curves <strong>of</strong> MV fuses and LV circuit-breaker<br />
8<br />
Minimum pre-arcing<br />
curve for 63 A HV fuses<br />
(current referred to the<br />
secondary side <strong>of</strong> the<br />
transformer)<br />
Isc maxi<br />
31.4 kA<br />
Step 4<br />
Step 3<br />
Step 2<br />
Step 1<br />
I
2<br />
3<br />
4<br />
<strong>Chapter</strong> J<br />
Protection against voltage surges<br />
in LV<br />
Contents<br />
<strong>General</strong> J2<br />
1.1 What is a voltage surge? J2<br />
1.2 The four voltage surge types J2<br />
1.3 Main characteristics <strong>of</strong> voltage surges J4<br />
1.4 Different propagation modes J5<br />
Overvoltage protection devices J6<br />
2.1 Primary protection devices (protection <strong>of</strong> <strong>installation</strong>s J6<br />
against lightning)<br />
2.2 Secondary protection devices (protection <strong>of</strong> internal J8<br />
<strong>installation</strong>s against lightning)<br />
Protection against voltage surges in LV J<br />
3.1 Surge protective device description J11<br />
3.2 Surge protective device standards J11<br />
3.3 Surge protective device data according to IEC 61643-1 standard J11<br />
3.4 Lightning protection standards J13<br />
3.5 Surge arrester <strong>installation</strong> standards J13<br />
Choosing a protection device J 4<br />
4.1 Protection devices according to the earthing system J14<br />
4.2 Internal architecture <strong>of</strong> surge arresters J15<br />
4.3 Installation <strong>rules</strong> J16<br />
4.4 Selection guide J17<br />
4.5 Choice <strong>of</strong> disconnector J22<br />
4.6 End-<strong>of</strong>-life indication <strong>of</strong> the surge arrester J22<br />
4.7 Application example: supermarket J23<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
J<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
J2<br />
J - Protection against voltage surges in LV<br />
<strong>General</strong><br />
. What is a voltage surge?<br />
A voltage surge is a voltage impulse or wave which is superposed on the rated<br />
network voltage (see Fig. J ).<br />
Voltage<br />
Fig. J1 : Voltage surge examples<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Lightning type impulse<br />
(duration = 100 µs)<br />
"Operating impulse"<br />
type dumped ring wave<br />
(F = 100 kHz to 1 MHz)<br />
This type <strong>of</strong> voltage surge is characterised by ( see Fig. J2):<br />
b The rise time (tf) measured in μs<br />
b The gradient S measured in kV/μs<br />
A voltage surge disturbs equipment and causes electromagnetic radiation.<br />
Furthermore, the duration <strong>of</strong> the voltage surge (T) causes a surge <strong>of</strong> energy in the<br />
<strong>electrical</strong> circuits which is likely to destroy the equipment.<br />
U max<br />
50 %<br />
Voltage (V or kV)<br />
Fig. J2 : Main overvoltage characteristics<br />
Rise time (tf)<br />
Voltage surge duration (T)<br />
.2 The four voltage surge types<br />
There are four types <strong>of</strong> voltage surges which may disturb <strong>electrical</strong> <strong>installation</strong>s and<br />
loads:<br />
b Atmospheric voltage surges<br />
b Operating voltage surges<br />
b Transient overvoltage at industrial frequency<br />
b Voltage surges caused by electrostatic discharge<br />
Atmospheric voltage surges<br />
Lightning risk – a few figures<br />
Between 2,000 and 5,000 storms are constantly forming around the earth. These<br />
storms are accompanied by lightning which constitutes a serious risk for both people<br />
and equipment. Strokes <strong>of</strong> lightning hit the ground at a rate <strong>of</strong> 30 to 100 strokes per<br />
second. Every year, the earth is struck by about 3 billion strokes <strong>of</strong> lightning.<br />
t<br />
Irms
J - Protection against voltage surges in LV<br />
Lightning comes from the discharge <strong>of</strong> <strong>electrical</strong><br />
charges accumulated in the cumulo-nimbus<br />
clouds which form a capacitor with the ground.<br />
Storm phenomena cause serious damage.<br />
Lightning is a high frequency <strong>electrical</strong><br />
phenomenon which produces voltage surges<br />
on all conductive elements, and especially on<br />
<strong>electrical</strong> loads and wires.<br />
<strong>General</strong><br />
b Throughout the world, every year, thousands <strong>of</strong> people are struck by lightning and<br />
countless animals are killed<br />
b Lightning also causes a large number <strong>of</strong> fires, most <strong>of</strong> which break out on farms<br />
(destroying buildings or putting them out <strong>of</strong> use)<br />
b Lightning also affects transformers, electricity meters, household appliances, and<br />
all <strong>electrical</strong> and electronic <strong>installation</strong>s in the residential sector and in industry.<br />
b Tall buildings are the ones most <strong>of</strong>ten struck by lightning<br />
b The cost <strong>of</strong> repairing damage caused by lightning is very high<br />
b It is difficult to evaluate the consequences <strong>of</strong> disturbance caused to computer or<br />
telecommunications networks, faults in PLC cycles and faults in regulation systems.<br />
Furthermore, the losses caused by a machine being put out <strong>of</strong> use can have financial<br />
consequences rising above the cost <strong>of</strong> the equipment destroyed by the lightning.<br />
Characteristics <strong>of</strong> lightning discharge<br />
Figure J3 shows the values given by the lighting protection committee (Technical<br />
Committee 81) <strong>of</strong> the I.E.C. As can be seen, 50 % <strong>of</strong> lightning strokes are <strong>of</strong> a force<br />
greater than 33 kA and 5 % are greater than 85 kA. The energy forces involved are<br />
thus very high.<br />
Beyond peak Current Gradient Total Number <strong>of</strong><br />
probability peak duration discharges<br />
P% I (kA) S (kA/μs) T (s) n<br />
95 7 9.1 0.001 1<br />
50 33 24 0.01 2<br />
5 85 65 1.1 6<br />
Fig. J3 : Lightning discharge values given by the IEC lightning protection committee<br />
It is important to define the probability <strong>of</strong> adequate protection when protecting a site.<br />
Furthermore, a lightning current is a high frequency (HF) impulse current reaching<br />
roughly a megahertz.<br />
The effects <strong>of</strong> lightning<br />
A lightning current is therefore a high frequency <strong>electrical</strong> current. As well as<br />
considerable induction and voltage surge effects, it causes the same effects as any<br />
other low frequency current on a conductor:<br />
b Thermal effects: fusion at the lightning impact points and joule effect, due to the<br />
circulation <strong>of</strong> the current, causing fires<br />
b Electrodynamic effects: when the lightning currents circulate in parallel conductors,<br />
they provoke attraction or repulsion forces between the wires, causing breaks or<br />
mechanical deformations (crushed or flattened wires)<br />
b Combustion effects: lightning can cause the air to expand and create overpressure<br />
which stretches over a distance <strong>of</strong> a dozen metres or so. A blast effect breaks windows<br />
or partitions and can project animals or people several metres away from their original<br />
position. This shock wave is at the same time transformed into a sound wave: thunder<br />
b Voltage surges conducted after an impact on overhead <strong>electrical</strong> or telephone lines<br />
b Voltage surges induced by the electromagnetic radiation effect <strong>of</strong> the lightning<br />
channel which acts as an antenna over several kilometres and is crossed by a<br />
considerable impulse current<br />
b The elevation <strong>of</strong> the earth potential by the circulation <strong>of</strong> the lightning current in the<br />
ground. This explains indirect strokes <strong>of</strong> lightning by step voltage and the breakdown<br />
<strong>of</strong> equipment<br />
Operating voltage surges<br />
A sudden change in the established operating conditions in an <strong>electrical</strong> network<br />
causes transient phenomena to occur. These are generally high frequency or<br />
damped oscillation voltage surge waves (see Fig. J1).<br />
They are said to have a slow gradient: their frequency varies from several ten to<br />
several hundred kilohertz.<br />
Operating voltage surges may be created by:<br />
b The opening <strong>of</strong> protection devices (fuse, circuit-breaker), and the opening or<br />
closing <strong>of</strong> control devices (relays, contactors, etc.)<br />
b Inductive circuits due to motors starting and stopping, or the opening <strong>of</strong><br />
transformers such as MV/LV substations<br />
b Capacitive circuits due to the connection <strong>of</strong> capacitor banks to the network<br />
b All devices that contain a coil, a capacitor or a transformer at the power supply<br />
inlet: relays, contactors, television sets, printers, computers, electric ovens, filters, etc.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
J3<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
J4<br />
J - Protection against voltage surges in LV<br />
Three points must be kept in mind:<br />
b A direct or indirect lightning stroke may<br />
have destructive consequences on <strong>electrical</strong><br />
<strong>installation</strong>s several kilometres away from<br />
where it falls<br />
b Industrial or operating voltage surges also<br />
cause considerable damage<br />
b The fact that a site <strong>installation</strong> is underground<br />
in no way protects it although it does limit the<br />
risk <strong>of</strong> a direct strike<br />
<strong>General</strong><br />
Transient overvoltages at industrial frequency (see Fig. J4)<br />
These overvoltages have the same frequency as the network (50, 60 or 400 Hz); and<br />
can be caused by:<br />
b Phase/frame or phase/earth insulating faults on a network with an insulated or<br />
impedant neutral, or by the breakdown <strong>of</strong> the neutral conductor. When this happens,<br />
single phase devices will be supplied in 400 V instead <strong>of</strong> 230 V.<br />
b A cable breakdown. For example, a medium voltage cable which falls on a low<br />
voltage line.<br />
b The arcing <strong>of</strong> a high or medium voltage protective spark-gap causing a rise in earth<br />
potential during the action <strong>of</strong> the protection devices. These protection devices follow<br />
automatic switching cycles which will recreate a fault if it persists.<br />
Normal voltage<br />
230/400 V<br />
Fig. J4 : Transient overvoltage at industrial frequency<br />
Voltage surges caused by <strong>electrical</strong> discharge<br />
In a dry environment, <strong>electrical</strong> charges accumulate and create a very strong<br />
electrostatic field. For example, a person walking on carpet with insulating soles<br />
will become <strong>electrical</strong>ly charged to a voltage <strong>of</strong> several kilovolts. If the person walks<br />
close to a conductive structure, he will give <strong>of</strong>f an <strong>electrical</strong> discharge <strong>of</strong> several<br />
amperes in a very short rise time <strong>of</strong> a few nanoseconds. If the structure contains<br />
sensitive electronics, a computer for example, its components or circuit boards may<br />
be damaged.<br />
.3 Main characteristics <strong>of</strong> voltage surges<br />
Figure J5 below sums up the main characteristics <strong>of</strong> voltage surges.<br />
Type <strong>of</strong> voltage surge Voltage surge Duration Front gradient<br />
coefficient or frequency<br />
Industrial frequency y 1.7 Long Industrial frequency<br />
(insulation fault) 30 to 1,000 ms (50-60-400 Hz)<br />
Operation 2 to 4 Short Average<br />
1 to 100 ms 1 to 200 kHz<br />
Atmospheric > 4 Very short Very high<br />
1 to 100 μs 1 to 1,000 kV/μs<br />
Fig. J5 : Main characteristics <strong>of</strong> voltage surges<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Transient overvoltage<br />
Normal voltage<br />
230/400 V<br />
t
J - Protection against voltage surges in LV<br />
<strong>General</strong><br />
.4 Different propagation modes<br />
Common mode<br />
Common mode voltage surges occur between the live parts and the earth:<br />
phase/earth or neutral/earth (see Fig. J6).<br />
They are especially dangerous for devices whose frame is earthed due to the risk <strong>of</strong><br />
dielectric breakdown.<br />
Fig. J6 : Common mode<br />
Differential mode<br />
Fig. J7 : Differential mode<br />
Ph<br />
Ph<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
N<br />
N<br />
Imc<br />
Voltage surge<br />
common mode<br />
Imd<br />
U voltage surge<br />
differential mode<br />
Imd<br />
Equipment<br />
Differential mode voltage surges circulate between live conductors: Phase to phase<br />
or phase to neutral (see Fig. J7). They are especially dangerous for electronic<br />
equipment, sensitive computer equipment, etc.<br />
Imc<br />
Equipment<br />
J5<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
J<br />
J - Protection against voltage surges in LV<br />
2 Overvoltage protection devices<br />
Two major types <strong>of</strong> protection devices are used to suppress or limit voltage surges:<br />
they are referred to as primary protection devices and secondary protection devices.<br />
2.1 Primary protection devices (protection <strong>of</strong><br />
<strong>installation</strong>s against lightning)<br />
The purpose <strong>of</strong> primary protection devices is to protect <strong>installation</strong>s against direct<br />
strokes <strong>of</strong> lightning. They catch and run the lightning current into the ground. The<br />
principle is based on a protection area determined by a structure which is higher<br />
than the rest.<br />
The same applies to any peak effect produced by a pole, building or very high<br />
metallic structure.<br />
There are three types <strong>of</strong> primary protection:<br />
b Lightning conductors, which are the oldest and best known lightning protection<br />
device<br />
b Overhead earth wires<br />
b The meshed cage or Faraday cage<br />
The lightning conductor<br />
The lightning conductor is a tapered rod placed on top <strong>of</strong> the building. It is earthed by<br />
one or more conductors (<strong>of</strong>ten copper strips) (see Fig. J8).<br />
Copper strip<br />
down conductor<br />
Test clamp<br />
Fig. J8 : Example <strong>of</strong> protection using a lightning conductor<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Crow-foot earthing
J - Protection against voltage surges in LV<br />
2 Overvoltage protection devices<br />
The <strong>design</strong> and <strong>installation</strong> <strong>of</strong> a lightning conductor is the job <strong>of</strong> a specialist.<br />
Attention must be paid to the copper strip paths, the test clamps, the crow-foot<br />
earthing to help high frequency lightning currents run to the ground, and the<br />
distances in relation to the wiring system (gas, water, etc.).<br />
Furthermore, the flow <strong>of</strong> the lightning current to the ground will induce voltage<br />
surges, by electromagnetic radiation, in the <strong>electrical</strong> circuits and buildings to be<br />
protected. These may reach several dozen kilovolts. It is therefore necessary to<br />
symmetrically split the down conductor currents in two, four or more, in order to<br />
minimise electromagnetic effects.<br />
Overhead earth wires<br />
These wires are stretched over the structure to be protected (see Fig. J9). They are<br />
used for special structures: rocket launch pads, military applications and lightning<br />
protection cables for overhead high voltage power lines (see Fig. J10).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
h<br />
Fig. J9 : Example <strong>of</strong> lightning protection using overhead earth wires<br />
Fig. J10 : Lightning protection wires<br />
Tin plated copper 25 mm 2<br />
Frame grounded earth belt<br />
i/2<br />
d > 0.1 h<br />
i/2<br />
i<br />
Metal post<br />
Lightning<br />
protection<br />
cables<br />
J<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
J8<br />
J - Protection against voltage surges in LV<br />
Primary lightning conductor protection devices<br />
such as a meshed cage or overhead earth<br />
wires are used to protect against direct strokes<br />
<strong>of</strong> lighting.These protection devices do not<br />
prevent destructive secondary effects on<br />
equipment from occurring. For example, rises<br />
in earth potential and electromagnetic induction<br />
which are due to currents flowing to the earth.<br />
To reduce secondary effects, LV surge arresters<br />
must be added on telephone and <strong>electrical</strong><br />
power networks.<br />
Secondary protection devices are classed in<br />
two categories: Serial protection and parallel<br />
protection devices.<br />
Serial protection devices are specific to a<br />
system or application.<br />
Parallel protection devices are used for: Power<br />
supply network, telephone network, switching<br />
network (bus).<br />
2 Overvoltage protection devices<br />
The meshed cage (Faraday cage)<br />
This principle is used for very sensitive buildings housing computer or integrated<br />
circuit production equipment. It consists in symmetrically multiplying the number <strong>of</strong><br />
down strips outside the building. Horizontal links are added if the building is high; for<br />
example every two floors (see Fig. J11). The down conductors are earthed by frog’s<br />
foot earthing connections. The result is a series <strong>of</strong> interconnected 15 x 15 m or<br />
10 x 10 m meshes. This produces better equipotential bonding <strong>of</strong> the building and<br />
splits lightning currents, thus greatly reducing electromagnetic fields and induction.<br />
Fig. J11 : Example <strong>of</strong> protection using the meshed cage (Faraday cage) principle<br />
2.2 Secondary protection devices (protection <strong>of</strong><br />
internal <strong>installation</strong>s against lightning)<br />
These handle the effects <strong>of</strong> atmospheric, operating or industrial frequency voltage<br />
surges. They can be classified according to the way they are connected in an<br />
<strong>installation</strong>: serial or parallel protection.<br />
Serial protection device<br />
This is connected in series to the power supply wires <strong>of</strong> the system to be protected<br />
(see Fig. J12).<br />
Power supply<br />
Fig. J12 : Serial protection principle<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Serial<br />
protection<br />
Installation to be protected<br />
Transformers<br />
They reduce voltage surges by inductor effect and make certain harmonics<br />
disappear by coupling. This protection is not very effective.<br />
Filters<br />
Based on components such as resistors, inductance coils and capacitors they<br />
are suitable for voltage surges caused by industrial and operation disturbance<br />
corresponding to a clearly defined frequency band. This protection device is not<br />
suitable for atmospheric disturbance.<br />
Up
J - Protection against voltage surges in LV<br />
2 Overvoltage protection devices<br />
Wave absorbers<br />
They are essentially made up <strong>of</strong> air inductance coils which limit the voltage surges,<br />
and surge arresters which absorb the currents. They are extremely suitable for<br />
protecting sensitive electronic and computing equipment. They only act against<br />
voltage surges. They are nonetheless extremely cumbersome and expensive.<br />
Network conditioners and static uninterrupted power supplies (UPS)<br />
These devices are essentially used to protect highly sensitive equipment, such as<br />
computer equipment, which requires a high quality <strong>electrical</strong> power supply. They<br />
can be used to regulate the voltage and frequency, stop interference and ensure a<br />
continuous <strong>electrical</strong> power supply even in the event <strong>of</strong> a mains power failure (for<br />
the UPS). On the other hand, they are not protected against large, atmospheric type<br />
voltage surges against which it is still necessary to use surge arresters.<br />
Parallel protection device<br />
The principle<br />
The parallel protection is adapted to any <strong>installation</strong> power level (see Fig. J13).<br />
This type <strong>of</strong> overvoltage protection is the most commonly used.<br />
Power supply<br />
Parallel<br />
protection<br />
Fig. J13 : Parallel protection principle<br />
Main characteristics<br />
b The rated voltage <strong>of</strong> the protection device must correspond to the network voltage<br />
at the <strong>installation</strong> terminals<br />
b When there is no voltage surge, a leakage current should not go through the<br />
protection device which is on standby<br />
b When a voltage surge above the allowable voltage threshold <strong>of</strong> the <strong>installation</strong><br />
to be protected occurs, the protection device abruptly conducts the voltage surge<br />
current to the earth by limiting the voltage to the desired protection level Up<br />
(see Fig. J14).<br />
Up<br />
U (V)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Up<br />
0 I (A)<br />
Fig. J14 : Typical U/I curve <strong>of</strong> the ideal protection device<br />
Installation to<br />
be protected<br />
When the voltage surge disappears, the protection device stops conducting and<br />
returns to standby without a holding current. This is the ideal U/I characteristic curve:<br />
b The protection device response time (tr) must be as short as possible to protect the<br />
<strong>installation</strong> as quickly as possible<br />
b The protection device must have the capacity to be able to conduct the energy<br />
caused by the foreseeable voltage surge on the site to be protected<br />
b The surge arrester protection device must be able to withstand the rated current In.<br />
J9<br />
© Schneider Electric - all rights reserved
J10<br />
© Schneider Electric - all rights reserved<br />
J - Protection against voltage surges in LV<br />
2 Overvoltage protection devices<br />
The products used<br />
b Voltage limiters<br />
They are used in MV/LV substations at the transformer output, in IT earthing scheme.<br />
They can run voltage surges to the earth, especially industrial frequency surges<br />
(see Fig. J15)<br />
Fig. J15 : Voltage limiter<br />
Overvoltage<br />
limiter<br />
b LV surge arresters<br />
This term <strong>design</strong>ates very different devices as far as technology and use are<br />
concerned. Low voltage surge arresters come in the form <strong>of</strong> modules to be installed<br />
inside LV switchboard. There are also plug-in types and those that protect power<br />
outlets. They ensure secondary protection <strong>of</strong> nearby elements but have a small flow<br />
capacity. Some are even built into loads although they cannot protect against strong<br />
voltage surges<br />
b Low current surge arresters or overvoltage protectors<br />
These protect telephone or switching networks against voltage surges from the<br />
outside (lightning), as well as from the inside (polluting equipment, switchgear<br />
switching, etc.)<br />
Low current voltage surge arresters are also installed in distribution boxes or<br />
built into loads.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
MV/LV<br />
PIM<br />
Permanent<br />
insulation<br />
monitor
J - Protection against voltage surges in LV<br />
3 Protection against voltage<br />
surges in LV<br />
3.1 Surge protective device description<br />
A surge protective device (SDP) is a device that limits transient voltage surges and<br />
runs current waves to ground to limit the amplitude <strong>of</strong> the voltage surge to a safe<br />
level for <strong>electrical</strong> <strong>installation</strong>s and equipment.<br />
The surge protective device includes one or several non linear components.<br />
The surge protective device eliminates voltage surges:<br />
b In common mode: Phase to earth or neutral to earth<br />
b In differential mode: Phase to phase or phase to neutral<br />
When a voltage surge exceeds the Uc threshold, the surge protective device (SDP)<br />
conducts the energy to earth in common mode. In differential mode the diverted<br />
energy is directed to another active conductor.<br />
The surge protective device has an internal thermal protection device which protects<br />
against burnout at its end <strong>of</strong> life. Gradually, over normal use after withstanding<br />
several voltage surges, the Surge Protective Device degrades into a conductive<br />
device. An indicator informs the user when end-<strong>of</strong>-life is close.<br />
Some Surge Protective Devices have a remote indication.<br />
In addition, protection against short-circuits is ensured by an external circuit-breaker.<br />
3.2 Surge protective device standards<br />
International standard IEC 61643-1 ed. 02/2005<br />
Surge protective devices connected to low-voltage power distribution systems.<br />
Three test classes are defined:<br />
b Class I tests: They are conducted using nominal discharge current (In), voltage<br />
impulse with 1.2/50 μs waveshape and impulse current Iimp.<br />
The class I tests is intended to simulate partial conducted lightning current impulses.<br />
SPDs subjected to class I test methods are generally recommended for locations<br />
at points <strong>of</strong> high exposure, e.g., line entrances to buildings protected by lightning<br />
protection systems.<br />
b Class II tests: They are conducted using nominal discharge current (In), voltage<br />
impulse with 1.2/50 μs waveshape<br />
b Class III tests: They are conducted using the combination waveform (1.2/50 and<br />
8/20 μs).<br />
SPDs tested to class II or III test methods are subjected to impulses <strong>of</strong> shorter<br />
duration. These SPDs are generally recommended for locations with lesser exposure.<br />
These 3 test classes cannot be compared, since each originates in a country and<br />
each has its own specificities. Moreover, each builder can refer to one <strong>of</strong> the 3 test<br />
classes.<br />
European standard EN 61643-11 2002<br />
Some requirements as per IEC 61643-1. Moreover SPDs are classified in three<br />
categories:<br />
Type 1: SPD tested to Class I<br />
Type 2: SPD tested to Class II<br />
Type 3: SPD tested to Class III<br />
3.3 Surge protective device data according to<br />
IEC 61643-1 standard<br />
b Surge protective device (SPD): A device that is intended to limit transient<br />
overvoltages and divert surge currents. It contains at least one nonlinear component.<br />
b Test classes: Surge arrester test classification.<br />
b In: Nominal discharge current; the crest value <strong>of</strong> the current through the SPD<br />
having a current waveshape <strong>of</strong> 8/20. This is used for the classification <strong>of</strong> the SPD for<br />
the class II test and also for preconditioning <strong>of</strong> the SPD for class I and II tests.<br />
b Imax: Maximum discharge current for class II test; crest value <strong>of</strong> a current through<br />
the SPD having an 8/20 waveshape and magnitude according to the test sequence<br />
<strong>of</strong> the class II operating duty test. Imax is greater than In.<br />
b Ic: Continuous operating current; current that flows in an SPD when supplied at<br />
its permament full withstand operating voltage (Uc) for each mode. Ic corresponds<br />
to the sum <strong>of</strong> the currents that flow in the SPD’s protection component and in all the<br />
internal circuits connected in parallel.<br />
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J12<br />
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J - Protection against voltage surges in LV<br />
Maxi<br />
100 %<br />
50 %<br />
1,2<br />
V<br />
Fig. J17 : 1.2/50 μs wave<br />
50<br />
t<br />
3 Protection against voltage<br />
surges in LV<br />
b Iimp: Impulse current, it is defined by a current peak value Ipeak and the charge<br />
Q. Tested according to the test sequence <strong>of</strong> the operating duty test. This is used for<br />
the classification <strong>of</strong> the SPD for class I test.<br />
b Un: Rated network voltage.<br />
b Uc: Maximum continuous operating voltage; the maximum r.m.s. or d.c. voltage<br />
which may be continuously applied to the SPDs mode <strong>of</strong> protection. This is equal to<br />
the rated voltage.<br />
b Up: Voltage protection level; a parameter that characterizes the performance <strong>of</strong><br />
the SPD in limiting the voltage across its terminals, which is selected from a list <strong>of</strong><br />
preferred values. This value shall be greater than the highest value <strong>of</strong> the measured<br />
limiting voltages.<br />
The most common values for a 230/400 V network are:<br />
1 kV - 1.2 kV - 1.5 kV - 1.8 kV - 2 kV - 2.5 kV.<br />
b Ures: Residual voltage, the peak value <strong>of</strong> the voltage that appears between the<br />
terminals <strong>of</strong> an SPD due to the passage <strong>of</strong> discharge current.<br />
The SPD is characterised by Uc, Up, In and Imax (see Fig. J16)<br />
b To test the surge arrester, standardized voltage and current waves have been<br />
defined that are specific to each country:<br />
v Voltage wave<br />
e.g. 1.2/50 μs (see Fig. J17)<br />
v Current wave<br />
Example 8/20 μs (see Fig. J18)<br />
Fig. J18 : 8/20 μs wave<br />
U p<br />
U c<br />
Maxi<br />
100 %<br />
50 %<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
U<br />
I<br />
< 1 mA<br />
Fig. J16 : Voltage/current characteristics<br />
8<br />
20<br />
I n I max<br />
v Other possible wave characteristics:<br />
4/10 μs, 10/1000 μs, 30/60 μs, 10/350 μs...<br />
Comparison between different surge protective devices must be carried out using the<br />
same wave characteristics, in order to get relevant results.<br />
t<br />
I
J - Protection against voltage surges in LV<br />
(1) According to IEC 60038<br />
(2) In Canada and USA for voltages to earth higher than 300 V,<br />
the impulse withstand voltage corresponding to the<br />
next higher voltage in column one applies.<br />
Category I is addressed to particular equipment engineering.<br />
Category II is addressed to product committees for equipment<br />
for connection to the mains.<br />
Category III is addressed to product committees <strong>of</strong> <strong>installation</strong><br />
material and some special product committees.<br />
Category IV is addressed to supply authorities and system<br />
engineers (see also 443.2.2).<br />
3 Protection against voltage<br />
surges in LV<br />
3.4 Lightning protection standards<br />
The IEC 62305 series (part 1 to 5) restructures and updates the publications <strong>of</strong><br />
IEC 61024 series, IEC 61312 series and IEC 61663 series.<br />
The need for protection, the economic benefits <strong>of</strong> installing protection measures and<br />
the selection <strong>of</strong> adequate protection measures should be determined in terms <strong>of</strong> risk<br />
management. Risk management is the subject <strong>of</strong> IEC 62305-2.<br />
The criteria for <strong>design</strong>, <strong>installation</strong> and maintenance <strong>of</strong> lightning protection measures<br />
are considered in three separate groups:<br />
b The first group concerning protection measures to reduce physical damage and life<br />
hazard in a structure is given in IEC 62305-3.<br />
b The second group concerning protection measures to reduce failures <strong>of</strong> <strong>electrical</strong><br />
and electronic systems in a structure is given in IEC 62305-4.<br />
b The third group concerning protection measures to reduce physical damage and<br />
failures <strong>of</strong> services connected to a structure (mainly <strong>electrical</strong> and telecommunication<br />
lines) is given in IEC 62305-5.<br />
3.5 Surge arrester <strong>installation</strong> standards<br />
b International: IEC 61643-12 selection and application principles<br />
b International: IEC 60364 Electrical <strong>installation</strong>s <strong>of</strong> buildings<br />
v IEC 60364-4-443: protection for safety<br />
When an <strong>installation</strong> is supplied by, or includes, an overhead line, a protection device<br />
against atmospheric overvoltages must be foreseen if the keraunic level <strong>of</strong> the site<br />
being considered corresponds to the external influences condition AQ 1 (more than<br />
25 days per year with thunderstorms).<br />
v IEC 60364-4-443-4: selection <strong>of</strong> equipment in the <strong>installation</strong>.<br />
This section helps with the choice <strong>of</strong> the protection level Up for the surge arrester in<br />
function <strong>of</strong> the loads to be protected.<br />
Rated residual voltage <strong>of</strong> protection devices must not be higher than the value in the<br />
voltage impulse withstand category II (see Fig. J19):<br />
Nominal voltage <strong>of</strong> Required impulse withstand voltage for<br />
the <strong>installation</strong> (1) V kV<br />
Three-phase Single-phase Equipment at Equipment <strong>of</strong> Appliances Specially<br />
systems (2) systems with the origin <strong>of</strong> distribution and protected<br />
middle point the <strong>installation</strong> final circuits equipment<br />
(impulse (impulse (impulse (impulse<br />
withstand withstand withstand withstand<br />
category IV) category III) category II) category I)<br />
120-240 4 2.5 1.5 0.8<br />
230/400 (2) - 6 4 2.5 1.5<br />
277/480 (2)<br />
400/690 - 8 6 4 2.5<br />
1,000 - Values subject to system engineers<br />
Fig. J19 : Choosing equipment for the <strong>installation</strong> according to IEC 60364<br />
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J - Protection against voltage surges in LV<br />
(1) These values are related to worst case fault conditions,<br />
therefore the tolerance <strong>of</strong> 10 % is not taken into account<br />
3 Protection against voltage<br />
surges in LV<br />
v IEC 60364-5-534: choosing and implementing <strong>electrical</strong> equipment<br />
This section describes surge arrester <strong>installation</strong> conditions:<br />
- According to earthing systems: The maximum continuous operating voltage Uc<br />
<strong>of</strong> SPDs shall be equal to or higher than shown in Fig. J20.<br />
SPDs connected<br />
between<br />
Line conductor and<br />
neutral conductor<br />
Each line conductor and<br />
PE conductor<br />
Neutral conductor and PE<br />
conductor<br />
Each line conductor and<br />
PEN conductor<br />
- At the origin <strong>of</strong> the <strong>installation</strong>: if the surge arrester is installed at the source <strong>of</strong><br />
an <strong>electrical</strong> <strong>installation</strong> supplied by the utility distribution network, its rated discharge<br />
current may be lower than 5 kA.<br />
If a surge arrester is installed downstream from an earth leakage protection device,<br />
an RCD <strong>of</strong> the s type, with immunity to impulse currents <strong>of</strong> less than 3 kA (8/20 μs),<br />
must be used.<br />
- Protection against overcurrent at 50 Hz and consequences <strong>of</strong> a SPD failure:<br />
protection against SPDs short-circuits is provided by the overcurrent protective<br />
devices F2 which are to be selected according to the maximum recommended rating<br />
for the overcurrent protective device given in the manufacturer's SPD instructions.<br />
- In the presence <strong>of</strong> lightning conductors: a surge arrester must be installed,<br />
additional specifications for surge arresters must be applied (see IEC 62305 part 4).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
System configuration <strong>of</strong> distribution network<br />
TT TN-C TN-S IT with<br />
distributed<br />
neutral<br />
IT without<br />
distributed<br />
neutral<br />
1.1 Uo NA 1.1 Uo 1.1 Uo NA<br />
1.1 Uo NA 1.1 Uo 3Uo (1) Line-to-line<br />
voltage (1)<br />
Uo (1) NA Uo (1) Uo (1) NA<br />
NA 1.1 Uo NA NA NA<br />
NA: not applicable<br />
NOTE 1: Uo is the line-to-neutral voltage <strong>of</strong> the low-voltage system.<br />
NOTE 2: This table is based on IEC 61643-1 amendment 1.<br />
Fig. J20 : Minimum required Uc <strong>of</strong> the SPD dependent on supply system configuration
J - Protection against voltage surges in LV<br />
4 Choosing a protection device<br />
When installing surge arresters, several elements must be considered, such as:<br />
b Cascading<br />
b Positioning with respect to residual current devices<br />
b The choice <strong>of</strong> disconnection circuit breakers<br />
The earthing system must also be taken into account.<br />
4.1 Protection devices according to the earthing<br />
system<br />
b Common mode overvoltage: basic protection involves the <strong>installation</strong> <strong>of</strong> a common<br />
mode surge arrester between phase and PE or phase and PEN, whatever type <strong>of</strong><br />
earthing system is used.<br />
b Differential mode overvoltage: in the TT and TN-S earthing systems, earthing the<br />
neutral leads to dissymmetry due to earthing impedances, which causes differential<br />
mode voltages to appear, whereas the overvoltage induced by a lightning strike is a<br />
common mode voltage.<br />
For example, let us consider a TT earthing system. A two-pole surge arrester is<br />
installed in common mode to protect the <strong>installation</strong> (see Fig. J21).<br />
Fig. J21 : Common mode protection only<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
I<br />
I<br />
The neutral earthing resistor R1 used for the pylons has a lower resistance than the<br />
earthing resistor R2 used for the <strong>installation</strong>. The lightning current will flow through<br />
circuit ABCD to earth via the easiest path. It will pass through varistors V1 and V2 in<br />
series, causing a differential voltage equal to twice the residual voltage <strong>of</strong> the surge<br />
arrester (Up1 + Up2) to appear at the terminals <strong>of</strong> A and C at the entrance to the<br />
<strong>installation</strong> in extreme cases.<br />
To protect the loads between Ph and N effectively, the differential mode voltage<br />
(between A and C) must be reduced.<br />
Another earthing system is therefore used (see Fig. J22).<br />
The lightning current flows through circuit ABH which has a lower impedance than<br />
circuit ABCD, as the impedance <strong>of</strong> the component used between B and H is null (gas<br />
filled spark gap).<br />
In this case, the differential voltage is equal to the residual voltage <strong>of</strong> the surge<br />
arrester (Up2).<br />
Fig. J22 : Common + differentiel mode protection<br />
I<br />
I<br />
I<br />
I<br />
I<br />
J15<br />
© Schneider Electric - all rights reserved
J16<br />
© Schneider Electric - all rights reserved<br />
J - Protection against voltage surges in LV<br />
PE<br />
Main earth terminal<br />
Fig. J26 : Connection example<br />
Earthing conductor<br />
4 Choosing a protection device<br />
Mode Between TT TN-S TN-C IT<br />
Differential phase and neutral yes yes - -<br />
Common phase and earth yes yes yes yes<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
phase and earth yes yes - yes (if distributed<br />
neutral)<br />
Fig. J23 : Connections to be made according to the earthing systems used, in the case <strong>of</strong><br />
atmospheric overvoltages<br />
4.2 Internal architecture <strong>of</strong> surge arresters<br />
b 2P, 3P, 4P surge arresters (see Fig. J24):<br />
v They provide protection against common-mode overvoltages only<br />
v They are appropriate for TN-C and IT earthing systems.<br />
Fig. J24 : 2P, 3P, 4P surge arresters<br />
b 1P+N, 3P+N surge arresters (see Fig. J25):<br />
v They provide protection against common-mode and differential-mode overvoltages<br />
v They are appropriate for TT, TN-S, and IT earthing systems.<br />
Fig. J25 : 1P+N, 3P+N surge arresters<br />
b Single-pole (1P) surge arresters (see Fig. J26):<br />
v They are used to satisfy the demand <strong>of</strong> different assemblies (according to the<br />
manufacturer’s instructions) by supplying only one product.<br />
However, special dimensioning will be required for N - PE protection<br />
(for example 1+N and 3P+N)<br />
v The assembly must be validated by means <strong>of</strong> the tests specified in EN 61643-11.
J - Protection against voltage surges in LV<br />
Cascading protection requires a minimum<br />
distance <strong>of</strong> at least 10 m between the two<br />
protection devices.<br />
This is valid, whatever the field <strong>of</strong> application:<br />
domestic, tertiary or industrial.<br />
Fig. J27 : Cascading <strong>of</strong> surge arresters<br />
4 Choosing a protection device<br />
4.3 Installation <strong>rules</strong><br />
The overvoltage protection study <strong>of</strong> an <strong>installation</strong> may show that the site is highly<br />
exposed and that the equipment to be protected is sensitive. The surge arrester<br />
must be able to discharge high currents and have a low level <strong>of</strong> protection. This dual<br />
constraint cannot always be handled by a single surge arrester. A second one will<br />
therefore be required (see Fig. J27).<br />
The first device, P1 (incoming protection) will be placed at the incoming end <strong>of</strong> the<br />
<strong>installation</strong>.<br />
Its purpose will be to discharge the maximum amount <strong>of</strong> energy to earth with a<br />
level <strong>of</strong> protection y 2000 V that can be withstood by the electrotechnical equipment<br />
(contactors, motors, etc.).<br />
The second device (fine protection) will be placed in a distribution enclosure, as<br />
close as possible to the sensitive loads. It will have a low discharge capacity and a<br />
low level <strong>of</strong> protection that will limit overvoltages significantly and therefore protect<br />
sensitive loads (y 1500 V).<br />
Coordination <strong>of</strong> surge arresters<br />
I<br />
The fine-protection device P2 is installed in parallel with the incoming protection<br />
device P1.<br />
If the distance L is too small, at the incoming overvoltage, P2 with a protection<br />
level <strong>of</strong> U2 = 1500 V will operate before P1 with a level <strong>of</strong> U1 = 2000 V. P2 will not<br />
withstand an excessively high current. The protection devices must therefore be<br />
coordinated to ensure that P1 activates before P2. To do this, we shall experiment<br />
with the length L <strong>of</strong> the cable, i.e. the value <strong>of</strong> the self-inductance between the two<br />
protection devices. This self-inductance will block the current flow to P2 and cause a<br />
certain delay, which will force P1 to operate before P2. A metre <strong>of</strong> cable gives a selfinductance<br />
<strong>of</strong> approximately 1μH.<br />
The rule ∆U= Ldi<br />
causes a voltage drop <strong>of</strong> approximately 100 V/m/kA, 8/20 μs<br />
dt<br />
wave.<br />
For L = 10 m, we get UL1 = UL2 ≈ 1000 V.<br />
To ensure that P2 operates with a level <strong>of</strong> protection <strong>of</strong> 1500 V requires<br />
U1 = UL1 + UL2 + U2 = 1000 + 1000 + 1500 V = 3500 V.<br />
Consequently, P1 operates before 2000 V and therefore protects P2.<br />
Note: if the distance between the surge arrester at the incoming end <strong>of</strong> the<br />
<strong>installation</strong> and the equipment to be protected exceeds 30 m, cascading the surge<br />
arresters is recommended, as the residual voltage <strong>of</strong> the surge arrester may rise<br />
to double the residual voltage at the terminals <strong>of</strong> the incoming surge arrester; as in<br />
the above example, the fine protection surge arrester must be placed as close as<br />
possible to the loads to be protected.<br />
The first rule to be observed is to ensure that the connection between the surge<br />
arrester and its disconnection circuit breaker does not exceed 50 cm.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
I<br />
Fig. J28 : Coordination <strong>of</strong> surge arresters<br />
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J18<br />
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J - Protection against voltage surges in LV<br />
1<br />
2<br />
3<br />
4 Choosing a protection device<br />
4.4 Selection guide<br />
Estimate the value <strong>of</strong> the equipment to be protected<br />
To estimate its value, consider:<br />
b The cost <strong>of</strong> the equipment in financial terms<br />
b The economic impact if the equipment goes down.<br />
b Domestic equipment:<br />
v audio-video, computers<br />
v household appliances<br />
v burglar alarm.<br />
b Sensitive equipment:<br />
v burglar alarm<br />
v fire alarm<br />
v access control<br />
v video surveillance.<br />
b Pr<strong>of</strong>essional equipment:<br />
v programmable machine<br />
v computer server<br />
v sound or light control system.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
b Building equipment:<br />
v automated heating or<br />
air-conditioning<br />
v lift.<br />
Determine the <strong>electrical</strong> architecture <strong>of</strong> buildings<br />
b Heavy equipment:<br />
v medical infrastructure<br />
v production infrastructure<br />
v heavy computer processing.<br />
Lightning protection can be calculated for an entire building or for part <strong>of</strong> a<br />
building that is <strong>electrical</strong>ly independent<br />
Depending on the size <strong>of</strong> the building and the extent <strong>of</strong> its <strong>electrical</strong> system, one or<br />
more surge arresters must be used in the various switchboards in the <strong>installation</strong>.<br />
b Detached house.<br />
b Apartment, small semi-detached house.<br />
b Communal part <strong>of</strong> a building.<br />
b Pr<strong>of</strong>essional premises.<br />
b Tertiary and industrial buildings:<br />
v single switchboard, main switchboard<br />
v distribution board<br />
v sensitive equipment more than 30 m from the switchboard.<br />
Understand the risk <strong>of</strong> the impact <strong>of</strong> lightning on the site<br />
Lightning is attracted by high points that conduct electricity. They can be:<br />
b Natural: tall trees, mountain crest, wet areas, ferrous soil<br />
b Artificial: chimney, aerial, pylon, lightning conductor.<br />
Indirect effects can be incurred within a fifty metre radius around the point <strong>of</strong> impact.<br />
Location <strong>of</strong> the building<br />
In an urban, peri-urban,<br />
grouped housing area.<br />
In an area where there is a<br />
particular hazard (pylon, tree,<br />
mountainous region, mountain<br />
crest, wet area or pond).<br />
In flat open country. In an exceptionally exposed area<br />
(lightning conductor on a building<br />
less than 50 metres away).
J - Protection against voltage surges in LV<br />
1<br />
Equipment to be<br />
protected<br />
2<br />
Determine the<br />
architecture <strong>of</strong> the<br />
building<br />
3<br />
Risk level <strong>of</strong><br />
the impact <strong>of</strong> a<br />
lightning strike<br />
Choice <strong>of</strong> type <strong>of</strong><br />
surge arrester<br />
Domestic equipment<br />
Type 2<br />
10 kA<br />
Detached house,<br />
Pr<strong>of</strong>essional<br />
premises<br />
Type 2<br />
40 kA<br />
Type 1<br />
25 kA<br />
+<br />
Type 2<br />
40 kA<br />
Audio-video, computers,<br />
household appliances,<br />
burglar alarm, etc.<br />
Type 1<br />
25 kA<br />
+<br />
Type 2<br />
40 kA<br />
Apartment, small<br />
semi-detached<br />
house<br />
Type 2<br />
10 kA<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Communal part <strong>of</strong> a<br />
building<br />
Note:<br />
Type 1: very high discharge capacity surge arrester used with a lightning conductor with an impact level <strong>of</strong> and .<br />
Type 2: surge arrester used in cascade behind a type 1 surge arrester or alone in zone and .<br />
Fig. J32 : Domestic equipment<br />
Lightning also propagates through<br />
telecommunications networks.<br />
It can damage all the equipment connected to<br />
these networks.<br />
4 Choosing a protection device<br />
Type 2<br />
40 kA<br />
Protection <strong>of</strong> telecommunications equipment<br />
Type 2<br />
65 kA<br />
Choice <strong>of</strong> surge arresters PRC<br />
Analogue telephone networks < 200 V b<br />
Type 1<br />
25 kA<br />
+<br />
Type 2<br />
40 kA<br />
Type 1<br />
25 kA<br />
+<br />
Type 2<br />
40 kA<br />
J19<br />
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J20<br />
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J - Protection against voltage surges in LV<br />
1<br />
Equipment to be<br />
protected<br />
2<br />
Determine the<br />
architecture <strong>of</strong> the<br />
building<br />
3<br />
Risk level <strong>of</strong><br />
the impact <strong>of</strong> a<br />
lightning strike<br />
Choice <strong>of</strong> type <strong>of</strong><br />
surge arrester<br />
Sensitive equipment: Building equipment:<br />
Type 2<br />
20 kA<br />
Single<br />
switchboard,<br />
main<br />
switchboard<br />
Type 2<br />
40 kA<br />
Type 2<br />
40 kA<br />
Burglar alarm,<br />
fire alarm,<br />
access control,<br />
video-surveillance, etc.<br />
Type 1<br />
25 kA<br />
or<br />
35 kA<br />
+<br />
Type 2<br />
40 kA<br />
Distribution<br />
board<br />
Type 2<br />
20 kA<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Automated heating or<br />
air-conditioning,<br />
lift, etc.<br />
Dedicated<br />
protection,<br />
more than<br />
30 m from a<br />
switchboard<br />
Note:<br />
Type 1: very high discharge capacity surge arrester used with a lightning conductor with an impact level <strong>of</strong> and .<br />
Type 2: surge arrester used in cascade behind a type 1 surge arrester or alone in zone and .<br />
Fig. J33 : Sensitive equipment, Building equipment<br />
4 Choosing a protection device<br />
Type 2<br />
8 kA
J - Protection against voltage surges in LV<br />
1<br />
Equipment to be<br />
protected<br />
2<br />
Determine the<br />
architecture <strong>of</strong> the<br />
building<br />
3<br />
Risk level <strong>of</strong><br />
the impact <strong>of</strong> a<br />
lightning strike<br />
Choice <strong>of</strong> type <strong>of</strong><br />
surge arrester<br />
Note:<br />
Type 1: very high discharge capacity surge arrester used with a lightning conductor with an impact level <strong>of</strong> and .<br />
Type 2: surge arrester used in cascade behind a type 1 surge arrester or alone in zone and .<br />
Fig. J34 : Pr<strong>of</strong>essional equipment<br />
Pr<strong>of</strong>essional equipment<br />
Type 2<br />
40 kA<br />
Single<br />
switchboard,<br />
main<br />
switchboard<br />
Type 2<br />
65 kA<br />
Type 2<br />
65 kA<br />
Programmable machine,<br />
server,<br />
sound or light control system,<br />
etc.<br />
Type 1<br />
25 kA<br />
or<br />
35 kA<br />
+<br />
Type 2<br />
40 kA<br />
4 Choosing a protection device<br />
Distribution<br />
board<br />
Type 2<br />
20 kA<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Dedicated<br />
protection,<br />
more than<br />
30 m from a<br />
switchboard<br />
Type 2<br />
8 kA<br />
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J22<br />
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J - Protection against voltage surges in LV<br />
1<br />
Equipment to be<br />
protected<br />
2<br />
Determine the<br />
architecture <strong>of</strong> the<br />
building<br />
3<br />
Risk level <strong>of</strong><br />
the impact <strong>of</strong> a<br />
lightning strike<br />
Choice <strong>of</strong> type <strong>of</strong><br />
surge arrester<br />
Heavy equipment<br />
Type 2<br />
65 kA<br />
Single<br />
switchboard,<br />
main<br />
switchboard<br />
Type 1<br />
25 kA<br />
+<br />
Type 2<br />
40 kA<br />
Type 1<br />
25 kA<br />
or<br />
35 kA<br />
+<br />
Type 2<br />
40 kA<br />
Medical, production,<br />
or heavy computer<br />
processing infrastructure,<br />
etc.<br />
Type 1<br />
25 kA<br />
or<br />
35 kA<br />
+<br />
Type 2<br />
40 kA<br />
Distribution<br />
board<br />
Type 2<br />
20 KA<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Dedicated<br />
protection,<br />
more than<br />
30 m from a<br />
switchboard<br />
Note:<br />
Type 1: very high discharge capacity surge arrester used with a lightning conductor with an impact level <strong>of</strong> and .<br />
Type 2: surge arrester used in cascade behind a type 1 surge arrester or alone in zone and .<br />
Fig. J35 : Heavy equipment<br />
Lightning can also propagate through<br />
telecommunications and computer networks.<br />
It can damage all the equipment connected<br />
to these networks: telephones, modems,<br />
computers, servers, etc.<br />
4 Choosing a protection device<br />
Type 2<br />
8 kA<br />
Protection <strong>of</strong> telecommunications and computer equipment<br />
Choice <strong>of</strong> surge arresters PRC PRI<br />
Analogue telephone networks < 200 V b<br />
Digital networks, analogue lines < 48 V<br />
Digital networks, analogue lines < 6 V<br />
VLV load supply < 48 V<br />
b<br />
b
J - Protection against voltage surges in LV<br />
4.5 Choice <strong>of</strong> disconnector<br />
The disconnector is necessary to ensure the safety <strong>of</strong> the <strong>installation</strong><br />
b One <strong>of</strong> the surge arrester parameters is the maximum current (Imax 8/20 μs<br />
wave) that it can withstand without degradation. If this current is exceeded, the surge<br />
arrester will be destroyed; it will be permanently short circuited and it is essential to<br />
replace it.<br />
The fault current must therefore be eliminated by an external disconnector installed<br />
upstream.<br />
The disconnector provides the complete protection required by a surge arrester<br />
<strong>installation</strong>, i.e.:<br />
v It must be able to withstand standard test waves:<br />
- it must not trip at 20 impulses at In<br />
- it can trip at Imax without being destroyed<br />
v the surge arrester disconnects if it short-circuits.<br />
b The ready-to-cable surge arresters with an integrated disconnection circuit breaker<br />
are:<br />
v Combi PRF1<br />
v Quick PF<br />
v Quick PRD.<br />
Surge arrester / disconnection circuit breaker correspondence<br />
table<br />
Types Isc Surge arresters 6 kA 10 kA 15 kA 25 kA 36 kA 50 kA 70 kA<br />
Type 1<br />
Type 2<br />
35 kA (1) PRF1 Master NH 160 A gL/gG fuse<br />
25 kA (1) PRF1 D125 NH 125 A gL/gG fuse<br />
65 kA (2) PF65, PRD65 C60N<br />
50 A<br />
Curve C<br />
40 kA (2) PF40, PRD40 C60N<br />
40 A<br />
Curve C<br />
20 kA (2) PF20, PRD20 C60N<br />
25 A<br />
Curve C<br />
8 kA (2) C60N<br />
20 A<br />
Curve C<br />
Isc: prospective short-circuit current at the point <strong>of</strong> <strong>installation</strong>.<br />
(1) Iimp.<br />
(2) Imax.<br />
Fig. J36 : Correspondence between surge arrester / disconnection circuit breaker<br />
4 Choosing a protection device<br />
NS160N 160 A NS160H<br />
160 A<br />
C60H<br />
50 A<br />
Curve C<br />
C60H<br />
40 A<br />
Curve C<br />
C60H<br />
25 A<br />
Curve C<br />
C60H<br />
20 A<br />
Curve C<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Contact us<br />
Contact us<br />
Contact us<br />
4.6 End-<strong>of</strong>-life indication <strong>of</strong> the surge arrester<br />
Various indication devices are provided to warn the user that the loads are no longer<br />
protected against atmospheric overvoltages.<br />
Type 1 surge arresters (with gas filled spark gap)<br />
PRF1 1P 260 V, Combi 1P+N and 3P+N and PRF1 Master<br />
These surge arresters have a light indicating that the module is in good working<br />
order. This indicator light requires a minimum operating voltage <strong>of</strong> 120 V AC.<br />
b The light does not come on:<br />
v if the operating voltage is y 120 V AC<br />
v if there is no network voltage<br />
v if the spark-over electronics are defective.<br />
J23<br />
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J24<br />
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J - Protection against voltage surges in LV<br />
Fig. J37 : Example <strong>of</strong> indication for PRD<br />
Fig. J39 : Example <strong>of</strong> indication for Quick PRD<br />
4 Choosing a protection device<br />
Type 2 surge arresters (varistor, varistor + gas filled spark gap)<br />
PF, PRD<br />
At end <strong>of</strong> life, the surge arrester or the cartridge are destroyed.<br />
b This can occur in two ways:<br />
v internal end-<strong>of</strong>-life disconnection: the accumulated electric shocks cause the<br />
varistors to age, resulting in an increase in leakage current.<br />
Above 1 mA, a thermal runaway occurs and the surge arrester disconnects.<br />
v external end-<strong>of</strong>-life disconnection: this occurs in the event <strong>of</strong> an excessive<br />
overvoltage (direct lightning strike on the line); above the discharge capacity <strong>of</strong> the<br />
surge arrester, the varistor(s) are dead short-circuited to earth (or possibly between<br />
phase and neutral). This short-circuit is eliminated when the mandatory associated<br />
disconnection circuit breaker opens.<br />
Quick PRD and Quick PF<br />
Whatever the hazards <strong>of</strong> the power supply network, Quick PRD and Quick PF<br />
incorporate a perfectly coordinated disconnector.<br />
b In the event <strong>of</strong> lightning strikes < Imax: like all surge arresters, they have internal<br />
anti-ageing protection.<br />
b In the event <strong>of</strong> a lightning strike > Imax: Quick PRD and Quick PF are selfprotected<br />
by their integrated disconnector.<br />
b In the event <strong>of</strong> neutral disconnection or phase-neutral reversal occurring on the<br />
power supply:<br />
Quick PRD and Quick PF are self-protected by their integrated disconnector.<br />
To simplify maintenance work, Quick PRD is fitted with local indicators and draw-out<br />
cartridges that are mechanically combined with the disconnector.<br />
Quick PRD has indicator lights on the cartridges and on the integrated disconnector,<br />
so that the work to be carried out can quickly be located.<br />
For safety reasons, the disconnector opens automatically when a cartridge is<br />
removed. It cannot be set until the cartridge is plugged in.<br />
When changing the cartridge, a phase/neutral failsafe system ensures that it can be<br />
plugged in safely.<br />
Operating state continuous display<br />
Quick PRD has an integrated reporting contact to send information about the<br />
operating state <strong>of</strong> the surge arrester from a remote location.<br />
Monitoring the surge arresters installed throughout the <strong>installation</strong> makes it possible<br />
to be continuously aware <strong>of</strong> their operating state and to ensure that the protection<br />
devices are always in good working order.<br />
b A reporting contact gives the alert:<br />
v at end <strong>of</strong> life <strong>of</strong> a cartridge<br />
v if a cartridge is missing, as soon as it has been removed<br />
v if a fault occurs on the line (short-circuit, neutral disconnection, phase-neutral<br />
reversal)<br />
v in the event <strong>of</strong> local manual operation (handle down).<br />
Quick PF has an optional indication reporting auxiliary (SR) that sends information<br />
about the operating state <strong>of</strong> the surge arrester from a remote location.<br />
4.7 Application example: supermarket<br />
Solutions and schematic diagram<br />
b The surge arrester selection guide has made it possible to determine the precise<br />
value <strong>of</strong> the surge arrester at the incoming end <strong>of</strong> the <strong>installation</strong> and that <strong>of</strong> the<br />
associated disconnection circuit breaker.<br />
b As the sensitive devices (Uimp < 1.5 kV) are located more than 30 m from the<br />
incoming protection device, the fine protection surge arresters must be installed as<br />
close as possible to the loads.<br />
b To ensure better continuity <strong>of</strong> service for cold room areas:<br />
v"si" type residual current circuit breakers will be used to avoid nuisance tripping<br />
caused by the rise in earth potential as the lightning wave passes through.<br />
b For protection against atmospheric overvoltages:<br />
v install a surge arrester in the main switchboard<br />
v install a fine protection surge arrester in each switchboard (1 and 2) supplying the<br />
sensitive devices situated more than 30 m from the incoming surge arrester<br />
v install a surge arrester on the telecommunications network to protect the devices<br />
supplied, for example fire alarms, modems, telephones, faxes.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
J - Protection against voltage surges in LV<br />
Fig. J40 : Telecommunications network<br />
4 Choosing a protection device<br />
Main<br />
switchboard<br />
C60<br />
20 A<br />
PRD<br />
8 kA<br />
C60<br />
40 A<br />
PRD<br />
40 kA<br />
MV/LV transformer<br />
160 kVA<br />
Switchboard 1 Switchboard<br />
2<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
ID<br />
"si"<br />
ID<br />
"si"<br />
Heating Lighting Freezer Refrigerator<br />
Storeroom lighting Power outlets<br />
Fig. J39 : Application example : supermarket<br />
Function <strong>of</strong> the surge arrester protection<br />
C60<br />
20 A<br />
PRD<br />
8 kA<br />
Fire-fighting system Alarm<br />
IT system Checkout<br />
b Conduct the lightning current to earth, ensuring a level <strong>of</strong> protection Up compatible<br />
with the <strong>electrical</strong> equipment to be protected.<br />
b Limit the rise in earth potential and the magnetic field induced.<br />
Cabling recommendations<br />
b Ensure the equipotentiality <strong>of</strong> the earth terminations <strong>of</strong> the building.<br />
b Reduce the looped power supply cable areas.<br />
Installation recommendations<br />
b Install a surge arrester, Imax = 40 kA (8/20 μs) and a C60 disconnection circuit<br />
breaker rated at 20 A.<br />
b Install fine protection surge arresters, Imax = 8 kA (8/20 μs) and the associated<br />
C60 disconnection circuit breakers rated at 20 A.<br />
J25<br />
© Schneider Electric - all rights reserved
2<br />
3<br />
4<br />
5<br />
<strong>Chapter</strong> K<br />
Energy Efficiency in <strong>electrical</strong><br />
distribution<br />
Contents<br />
Introduction K2<br />
Energy efficiency and electricity K3<br />
2.1 The world is now ready for energy-efficient actions and programs. K3<br />
2.2 A new challenge: <strong>electrical</strong> data K4<br />
One process, several players K5<br />
3.1 Energy Efficiency needs an Enterprise approach K5<br />
3.2 Economic competitiveness study K6<br />
3.3 The varied pr<strong>of</strong>iles and missions <strong>of</strong> players in the company K8<br />
From <strong>electrical</strong> measurement to <strong>electrical</strong> information K 0<br />
4.1 Physical value acquisition K10<br />
4.2 Electrical data for real objectives K12<br />
4.3 Measurement starts with the "stand alone product" solution K13<br />
Communication and Information System k 6<br />
5.1 Communication network at product, equipment and site level<br />
5.2 From Network Monitoring and Control System<br />
K16<br />
to Intelligent Power Equipment K19<br />
5.3 e-Support becomes accessible K21<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
K<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
K2<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Certain information in this chapter is taken<br />
from guides published by Carbon Trust<br />
(www.carbontrust.co.uk) GPG119 and GPG231.<br />
Introduction<br />
Power monitoring and control system may be <strong>of</strong> high benefice for the owner <strong>of</strong> an<br />
<strong>electrical</strong> network as a strategic piece in the global “Energy Efficiency” approach.<br />
Calculating Total Cost <strong>of</strong> Ownership (TCO) <strong>of</strong> an <strong>electrical</strong> network not only includes<br />
the initial equipment investment but also its economic performance in operation.<br />
Safety staff, the <strong>electrical</strong> billing manager, the chief site electrician or the facility<br />
manager, are all becoming increasingly concerned. The pr<strong>of</strong>iles vary, but each <strong>of</strong><br />
these people's mission includes careful management <strong>of</strong> electricity, its procurement<br />
and the network that distributes it.<br />
Fewer expensive power outages for the company’s business, less consumption<br />
wastage, no more maintenance operations than necessary, these are the objectives<br />
that a decision making assistance system focused on Energy Efficiency must satisfy<br />
and make available to each person, whatever their pr<strong>of</strong>ile.<br />
Nowadays, entering the “Energy Efficiency” approach doesn’t mean setting-up a<br />
complex and expensive system. Some simple features are really affordable with a<br />
very good payback because they can be directly embedded in the power equipment.<br />
Once the <strong>electrical</strong> <strong>installation</strong> is equipped with measurement functions, it can share<br />
the communication medium <strong>of</strong> the user’s Intranet site. In addition operation won’t<br />
need specific skills and training. It will only require the use <strong>of</strong> license-free s<strong>of</strong>tware<br />
such as Intranet browsers.<br />
Upgradeability or e-services through Internet are also now a reality, based on new<br />
technologies that come from the Office and Communication world. Then being in<br />
a position <strong>of</strong> taking advantages <strong>of</strong> these new possibilities will be more and more a<br />
differentiating behavior.<br />
Forecasting - Anticipation<br />
Extensions<br />
Improvement<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
New Design Retr<strong>of</strong>it<br />
End <strong>of</strong> life<br />
Design <strong>installation</strong><br />
Maintenance optimization<br />
MV<br />
LV<br />
ASI<br />
Sheddable Offices<br />
HVAC<br />
MV<br />
LV<br />
MLVS 1 MLVS 2<br />
PROCESS<br />
Trunking<br />
Operation<br />
e-services
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
2 Energy efficiency and electricity<br />
2.1 The world is now ready for energy-efficient<br />
actions and programs.<br />
The first big movement was initiated by the Kyoto protocol in 1997, updated in 2006.<br />
This well known world wide agreement requires participating countries to collectively<br />
reduce greenhouse gas emissions to an annual average <strong>of</strong> about 5 percent below<br />
the 1990 level over the 2008-2012 period.<br />
The protocol is based on three primary market mechanisms:<br />
b The Clean Development Mechanism (CDM), arrangement for reductions to be<br />
"sponsored" in countries not bound by emission targets<br />
b Joint Implementation, program that allows industrialized countries to meet part<br />
<strong>of</strong> their required cuts in greenhouse-gas emissions by implementing projects that<br />
reduce emissions in other countries.<br />
b Emissions trading, mechanism through which Parties with emission commitments<br />
may trade units <strong>of</strong> their emission allowances with other Parties because they are<br />
ahead <strong>of</strong> their target. This is the so called “carbon market”.<br />
All geographic areas at country, regional and federal level have launched<br />
programs, actions, regulations:<br />
b regulations and standards enforced in Europe (Fig. K1),<br />
b vision and strong initiatives in Asia.<br />
b strong programs in the US<br />
Fig. K1 : European parliament and counsel directive 2006/32/CE dated 5 April 2006 relative to<br />
the energy efficiency for end users and energy services<br />
ISO 14001 that defines principles and processes to permanently reduce energy consumption<br />
and waste emission in any organization.<br />
Drivers to develop energy efficiency programs – especially on the <strong>electrical</strong> form <strong>of</strong><br />
energy - are getting stronger and stronger. Energy Efficiency plan is at the top <strong>of</strong> the<br />
agenda for a growing number <strong>of</strong> companies:<br />
b Buildings are the biggest energy consumers and a priority target,<br />
b With cost <strong>of</strong> energy multiplied by 2 in the last 3 years, electricity saving is<br />
becoming a significant source <strong>of</strong> productivity gain for the industry,<br />
b Saving energy is now a part <strong>of</strong> the Corporate Social Responsibility commitment <strong>of</strong><br />
most listed companies,<br />
b With production and distribution networks under increased pressure from rising<br />
demand and scarce resources, availability <strong>of</strong> electricity is a rising concern for<br />
Industry heavily impacted by the consequences <strong>of</strong> outages,<br />
b The residential sector is a key sector and more and more impacted.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
K<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
K<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Fig. K2 : The Schneider Electric Solutions for Power & Control<br />
2 Energy efficiency and electricity<br />
2.2 A new challenge: <strong>electrical</strong> data<br />
All <strong>of</strong> the features <strong>of</strong> the current developments lead to the appearance <strong>of</strong> a “New<br />
Electrical World” in which the key considerations will be:<br />
b controlling risks related to power outages<br />
b energy yield or efficiency and control <strong>of</strong> costs: MWh price increased between 2003<br />
and 2006 from 30€ up to 60€ for deregulated markets in Europe<br />
b renewable energy<br />
b the environment and sustainable development.<br />
Electricity usage will become smarter and more rational to contribute both to the<br />
competitiveness <strong>of</strong> companies, their energy independence and protection <strong>of</strong> the<br />
environment. These new ground-<strong>rules</strong> mean that corporate decision makers have<br />
to implement new resources, and in particular products and services to accompany<br />
electricity consumers in this approach.<br />
In particular, the setting up <strong>of</strong> a global information system in the company will allow<br />
comprehensive <strong>electrical</strong> performance data to be streamed, in real time and remotely<br />
for (Fig. K2):<br />
b Predicting <strong>electrical</strong> network non-availability,<br />
b Recording <strong>electrical</strong> quality,<br />
b Optimizing consumption per building, sector, unit, workshop, site, excessive<br />
consumption or abnormal variations. We will therefore have all <strong>of</strong> the data required<br />
to make direct savings on electricity billing. End users can therefore take advantage<br />
<strong>of</strong> <strong>electrical</strong> network monitoring to avoid any wastage and to supply energy where it<br />
is really necessary.<br />
b Organizing <strong>electrical</strong> equipment maintenance.<br />
b Better purchasing <strong>of</strong> <strong>electrical</strong> energy and in certain cases, better resale.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Fig. K3 : Step by step approach to organizing energy<br />
management<br />
Level<br />
4<br />
2<br />
0<br />
b Gain commitment<br />
b Identify stakeholder needs<br />
b Establish policy<br />
b Set objectives and targets<br />
b Prepare action plan<br />
b Allocate roles and<br />
responsabilities<br />
b Prioritise investments<br />
b Train<br />
b Consider business<br />
integration and barriers<br />
to implementation<br />
b Audit process<br />
b Distribute audit findings<br />
3 One process, several players<br />
3.1 Energy Efficiency needs an Enterprise approach<br />
An information system must be integrated in a global approach in the company.<br />
The following step by step approach to organizing energy management (as shown<br />
in figure K1) is a structured method for managing projects and achieving results. It<br />
can be applied to very simple as well as complex tasks and has proved itself to be<br />
both robust and practical.<br />
Refer to 6 Sigma philosophy - Define, Measure, Analyze, Improve, Control - you<br />
cannot fix what you do not measure.<br />
Gain commitment<br />
In order to achieve action towards lasting energy efficiency, it is essential to gain the<br />
commitment <strong>of</strong> the most senior members <strong>of</strong> the management team as individuals<br />
and part <strong>of</strong> the corporate body.<br />
Understanding begins with:<br />
b learning about current energy consumption levels and costs<br />
b mapping the ways in which energy is used<br />
b determining the standards for efficient consumption in the organization<br />
b analyzing the possibilities for saving costs through reducing energy consumption<br />
so that realistic targets can be set<br />
b recognizing the environmental effects <strong>of</strong> energy consumption.<br />
Plan and organize<br />
The first step should be to produce a suitable energy policy for the organization. By<br />
developing and publishing such a policy, senior managers promote their commitment<br />
to achieving excellence in energy management. They should do this in a way that<br />
harnesses the culture <strong>of</strong> the organization to best effect.<br />
Implement<br />
Everyone must have some involvement in implementing the energy policy. However,<br />
to facilitate a structured approach, start by assigning special responsibilities to some<br />
individuals and groups.<br />
Control and monitor<br />
Each project should have an owner – an individual or a team with overall<br />
responsibility for monitoring efforts and steering it to a successful conclusion. Again<br />
Information System linked to <strong>electrical</strong> energy use and its impact on the core activity<br />
<strong>of</strong> the company will support the owner’s actions.<br />
Senior executives should underline the importance <strong>of</strong> projects by requiring regular<br />
progress reports, and by publicizing and endorsing success, which can further<br />
support individual motivation and commitment.<br />
The energy management matrix:<br />
Energy policy Organising Motivation Informations systems Marketing Investment<br />
Energy policy, action<br />
plan and regular review<br />
have commitment <strong>of</strong> top<br />
management as part<br />
<strong>of</strong> an environmental<br />
strategy<br />
Unadopted energy policy<br />
set by energy manager<br />
or senior departmental<br />
manager<br />
Get<br />
commitment<br />
Understand<br />
Plan and<br />
organize<br />
Implement<br />
Control and<br />
monitor<br />
Energy management<br />
fully integrated into<br />
management structure.<br />
Clear delegation <strong>of</strong><br />
responsability for energy<br />
consumption<br />
Energy manager in<br />
post, reporting to<br />
ad-hoc committee, but<br />
line management and<br />
authority are unclear<br />
No explicit policy No energy management<br />
or any format delegation<br />
<strong>of</strong> responsibility for<br />
energy consumption<br />
Formal and informal Comprehensive system<br />
channels <strong>of</strong><br />
sets targets, monitors<br />
communication regularity consumption, identifies<br />
exploited by energy faults, quantifies savings<br />
manager and energy staff and provides budget<br />
at all levels<br />
tracking<br />
Contact with major<br />
users through ad-hoc<br />
committee chaired by<br />
senior departmental<br />
manager<br />
Monitoring and targeting<br />
reports based on supply<br />
meter data. Energy unit<br />
has ad-doc involvement<br />
in budget setting<br />
No contact with users No information system.<br />
No accounting for energy<br />
consumption<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Marketing the value <strong>of</strong><br />
energy efficiency and the<br />
performance <strong>of</strong> energy<br />
management both within<br />
the organisation and<br />
outside it<br />
Some ad-doc staff<br />
awareness training<br />
No promotion <strong>of</strong> energy<br />
efficiency<br />
Positive discrimination<br />
in favour <strong>of</strong> "green"<br />
schemes with detailed<br />
investment appraisal<br />
<strong>of</strong> all new-build<br />
and refurbishment<br />
opportunities<br />
Investment using short<br />
term pay back criteria<br />
only<br />
No investment in<br />
increasing energy<br />
efficiency in premises<br />
K<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
K<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
3.2 Economic competitiveness study<br />
Company data 00000 Automatic calculation Contributing factors Savings per Item<br />
Fig. K4 : Example for calculating the return on investment<br />
3 One process, several players<br />
An information system on energy efficiency related to <strong>electrical</strong> usage must also<br />
be looked at in terms <strong>of</strong> an economic study to ensure the growth <strong>of</strong> economic<br />
competitiveness.<br />
This study basically depends on allocating financial value to electricity consumption,<br />
to operating losses related to the non-availability <strong>of</strong> energy and to maintenance costs<br />
in order to better manage the <strong>electrical</strong> <strong>installation</strong>.<br />
Preliminary stage: review the current situation and build a financial study<br />
(Figure K4).<br />
The need for a measurement <strong>installation</strong> is justified by the gains that this generates.<br />
A solution that covers the full <strong>installation</strong> represents a major improvement in the<br />
company’s competitiveness, but it requires the team concerned to actually use this<br />
capacity.<br />
Example :<br />
The figure below is an example for calculating the return on investment – available in<br />
Excel on www.transparentready.com.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Savings / Investment per<br />
category<br />
Total savings or<br />
investment<br />
Background: your organisation’s characteristics<br />
Annual revenues 100,000,000<br />
Net pr<strong>of</strong>it (%) 10 %<br />
Annual work hours (hours/day x days/week x weeks/year) 1.950 hrs<br />
Average hourly wage (loaded rate) 75<br />
Annual <strong>electrical</strong> energy costs 1,000,000<br />
Interest rate 15 %<br />
Corporate tax rate 30 %<br />
Annual energy cost savings potential<br />
Reduction in energy usage (% estimated) 10 %<br />
Reduction in energy usage 100,000<br />
Reduction in demand charges 20,000<br />
Power factor penalties avoided 20,000<br />
Energy billing errors avoided 5,000<br />
Energy costs allocated to tenants 0<br />
Annual energy cost savings 145,000<br />
Downtime cost avoidance potential<br />
Number <strong>of</strong> downtime events per year 2<br />
Hours <strong>of</strong> downtime per event 1.5 hrs<br />
Hours to recovery per downtime event 2 hrs<br />
Employees idled per downtime event 250<br />
Manufacturing employees required for line start-up 10<br />
IS employees required for computer system recovery 2<br />
Reduction in equipment replacements (e.g., transformers) 25,000<br />
Reduction in scrapped products or parts 50,000<br />
Corporate pr<strong>of</strong>it increase 15,385<br />
Increase in productive work hours 56,250<br />
Reduction in computer system recovery hours 600<br />
Reduction in manufacturing line start-up costs 3,000<br />
Annual downtime cost avoidance 150,235<br />
Operations & maintenance savings potential<br />
Employees assigned to manually read meters 3<br />
Employees assigned to maintenance 2<br />
Employees assigned to energy data analysis 2<br />
Activity-based costing savings (e.g., equipment or process removal) 50,000<br />
Equipment maintenance savings 10,000<br />
Automatic meter reading 7,875<br />
Fewer maintenance inspections 2,250<br />
Fewer hours for data analysis 10,500<br />
Operations & maintenance savings 80,625<br />
Total annual gross savings potential 375,860<br />
Transparent Ready system investment<br />
Number <strong>of</strong> buildings where energy is to be managed 2<br />
Metering devices, main/critical feeders, per building 10<br />
Metering devices, non-critical feeders, per building 15<br />
Metering devices, simple energy usage, per building 15<br />
Device costs 125,000<br />
S<strong>of</strong>tware costs 15,000<br />
Computer equipment costs 8,000<br />
Installation 160,000<br />
Configuration 8,000<br />
Training 3,500<br />
Support contract 14,338<br />
Total system investment 333,838<br />
ROI summary<br />
Invested capital -333,838<br />
Gross annual savings 375,860<br />
Yearly depreciation -66,768<br />
Corporate tax -112,758<br />
Net annual savings (after taxes and depreciation) 196,334<br />
Payback period (before tax & dep) (in months) 11<br />
Payback period (after tax & dep) (in months) 20<br />
Net present value 324,304<br />
Discounted return on investment (NPV / Invested Capital) 97 %
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Invest in three steps<br />
1- Formulate priorities<br />
2- Define key <strong>electrical</strong> values<br />
3- Select components<br />
3 One process, several players<br />
Step 1 : formulate priorities<br />
Each industrial or tertiary site has its own requirements and a specific <strong>electrical</strong><br />
distribution architecture. According to the site’s requirements, determine the<br />
appropriate energy efficiency applications (Figure K ):<br />
Objective Application<br />
Consumption optimization Cost allocation<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Energy usage analysis<br />
Pumps & fans for Industry & Infrastructure<br />
Pumps & fans for Buildings<br />
Lighting control<br />
Energy purchasing optimization Peak demand reduction<br />
Improving the efficiency <strong>of</strong><br />
teams in charge <strong>of</strong> <strong>electrical</strong><br />
<strong>installation</strong> operation<br />
Improving energy availability<br />
and quality<br />
Electricity procurement optimization<br />
Sub-billing<br />
Electrical Distribution alarming and event logging<br />
Electrical Distribution network remote control<br />
Electrical Distribution network automation<br />
Asset optimization Statistical analysis <strong>of</strong> equipment usage - Power Factor<br />
Correction<br />
Fig. K5 : Objective and application<br />
Step 2 : define key <strong>electrical</strong> values<br />
b once we have formulated the priorities, we can define the key <strong>electrical</strong> values to<br />
be included in the measurement system<br />
b the parameters to take into account must allow us to detect a disturbance or a<br />
phenomenon as soon as it appears, in other words before it has a detrimental effect<br />
to the <strong>electrical</strong> <strong>installation</strong> and its current consumers<br />
b the method includes installing an appropriate device on each feeder concerned so<br />
as to be as ready as possible for requirements, and another at the site <strong>installation</strong><br />
head so as to have an overview. However, we also need to identify vital feeders for<br />
the company’s business and feeders on costly processes so as to take account <strong>of</strong><br />
this information in the solution.<br />
Example: if the application consumes a lot <strong>of</strong> electricity and is not sensitive to quality, the<br />
metering system involves the appropriate measurement products. In the same way, a highly<br />
sensitive application in terms <strong>of</strong> energy quality requires a different type <strong>of</strong> metering product.<br />
Step 3 : select components<br />
For existing <strong>installation</strong>s: some <strong>of</strong> your <strong>electrical</strong> equipment already includes<br />
measurement products.<br />
Example: protection relays <strong>of</strong>ten include measurement functions. You simply have to make them<br />
communicate via a Modbus series link to the intranet site.<br />
K<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
K<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Pr<strong>of</strong>ile Org Skill Role Data display When? Data format<br />
Security staff Site No specific technical<br />
<strong>electrical</strong> skills.<br />
Maintenance<br />
Manager<br />
Safety <strong>of</strong> people and<br />
property.<br />
Via an alarm screen<br />
in the central security<br />
station. By DECT*, GSM<br />
or general circulation.<br />
Site People management, With his team, ensuring MMS/SMS, PC on<br />
overall <strong>electrical</strong> network the correct technical Intranet, email.<br />
skills, has been in his operation in all areas<br />
position for 3 to 8 years, (refrigeration, air<br />
technician level with conditioning, electricity,<br />
strong decision making security, public safety<br />
independence. Delegates etc.). Priority is given<br />
electrotechnical problems to availability, he is<br />
to outside organizations challenged on overheads<br />
(e.g.: calculating and therefore on <strong>electrical</strong><br />
protection settings). consumption, decides<br />
on the involvement <strong>of</strong><br />
outside companies and<br />
contributes to investment<br />
dossiers.<br />
Site Manager Site Competency in corporate<br />
management and in<br />
executive management.<br />
EE Manager for<br />
a multinational<br />
company<br />
Site/HQ Buyer/ global energy<br />
purchasing contract<br />
negotiator.<br />
Fig. K6 : The varied pr<strong>of</strong>iles and missions <strong>of</strong> players in the company<br />
3 One process, several players<br />
3.3 The varied pr<strong>of</strong>iles and missions <strong>of</strong> players in<br />
the company<br />
The setting up <strong>of</strong> an information system allows access to important data from<br />
<strong>electrical</strong> equipment and must involve staff with a IT and <strong>electrical</strong> knowledge pr<strong>of</strong>ile<br />
which by definition is very varied in the company. (Figures K and K ).<br />
Example : the table below shows a few examples <strong>of</strong> the pr<strong>of</strong>iles in a hypermarket. There are<br />
others such as Facility Management staff, workshop production managers or factory production<br />
managers.<br />
Responsible for a<br />
pr<strong>of</strong>it centre. Ensures<br />
compliance with<br />
procedures by staff via a<br />
management chart with<br />
performance indicators.<br />
Challenged on margin<br />
and turnover and<br />
therefore on overheads.<br />
Responsible for the<br />
global energy bill for the<br />
company via subsidiaries<br />
throughout the world and<br />
challenges entities with<br />
one another.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Rarely, on event Application order for<br />
planned procedures<br />
according to the type <strong>of</strong><br />
<strong>electrical</strong> event and a<br />
warning to site managers<br />
according to a predefined<br />
list.<br />
Rarely, on event, periodic<br />
consultation <strong>of</strong> reports,<br />
frequent consultation <strong>of</strong><br />
information on request.<br />
The data is shared with<br />
his team:<br />
- measurement screens<br />
with assistance as to<br />
possible interpretation<br />
(limits etc.)<br />
- consumption screens<br />
(KWh and Euro),<br />
- time-stamped events,<br />
- address book for outside<br />
players,<br />
- <strong>electrical</strong> single-line<br />
diagram <strong>of</strong> the site,<br />
drawings <strong>of</strong> <strong>electrical</strong><br />
cabinets and a link to<br />
manufacturing notices,<br />
- financial report, data<br />
used for the investment<br />
dossier,<br />
- indicators to be filled<br />
in on <strong>electrical</strong> network<br />
performance.<br />
Economic report Monthly Financial aspects<br />
including <strong>electrical</strong><br />
consumption, the link<br />
between the turnover<br />
generating business<br />
and electricity, the cost<br />
<strong>of</strong> maintenance <strong>of</strong> the<br />
<strong>electrical</strong> network.<br />
Economic report Monthly Financial features<br />
including <strong>electrical</strong><br />
consumption for each <strong>of</strong><br />
the multinational entities.
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
- Major faults<br />
- Security<br />
- Shop<br />
Digital<br />
inputs<br />
- Major faults<br />
- Minor faults<br />
- Maintenance<br />
- Shop<br />
Communication<br />
Modbus<br />
Shop i<br />
PSTN<br />
Meters<br />
for kWh<br />
and kVAh<br />
- Data collection<br />
- Local communication to users (critical)<br />
- Communication to data Centre<br />
3 One process, several players<br />
- kWh<br />
- kWh1<br />
- kWh2<br />
- kWh3<br />
- Tariff alarm<br />
- Major faults<br />
- Minor faults<br />
- Tech. Mngr.<br />
- Shop<br />
- Finance shop<br />
- Major faults<br />
- Shop Mngr.<br />
- Shop<br />
Fig. K7 : Example: configuration <strong>of</strong> a shopping centre with various players in place<br />
- Finance<br />
shops<br />
- Cost CTRL<br />
- Country<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
WEB<br />
- Finance<br />
shops<br />
- Cy Mngr.<br />
- Country<br />
Data<br />
center<br />
- Tariff structure<br />
- Pr<strong>of</strong>iles Mngt<br />
- Bills computation<br />
- Compounded data<br />
- Report generation & mailing<br />
- Storage<br />
- kWh shops<br />
- Finance shops<br />
- All (Pulled)<br />
- Energy Mngr.<br />
- Country<br />
- Kwh Cy<br />
- Finance<br />
Country<br />
- Finance Cies<br />
- Cost CTRL<br />
- Corporate<br />
Country i, n shops<br />
- Energy<br />
purchaser<br />
- Country<br />
K<br />
© Schneider Electric - all rights reserved
K10<br />
© Schneider Electric - all rights reserved<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
4 From <strong>electrical</strong> measurement to<br />
<strong>electrical</strong> information<br />
The energy efficiency performance in terms <strong>of</strong> electricity can only be expressed in<br />
terms <strong>of</strong> fundamental physical measurements – voltage, current, harmonics, etc.<br />
These physical measurements are then reprocessed to become digital data and then<br />
information.<br />
In the raw form, data are <strong>of</strong> little use. Unfortunately, some energy managers become<br />
totally immersed in data and see data collection and collation as their primary task.<br />
To gain value from data they must be transformed into information (used to support<br />
the knowledge development <strong>of</strong> all those managing energy) and understanding (used<br />
to action energy savings).<br />
The operational cycle is based on four processes: data collection; data analysis;<br />
communication; and action (Fig. K8). These elements apply to any information<br />
system. The cycle works under condition that an adequate communication network<br />
has been set up.<br />
Data analysis<br />
(data to information)<br />
Fig. K8 : The operational cycle<br />
The data processing level results in information that can be understood by the<br />
recipient pr<strong>of</strong>ile: the ability to interpret the data by the user remains a considerable<br />
challenge in terms <strong>of</strong> decision making.<br />
The data is then directly linked to loads that consume electricity – industrial process,<br />
lighting, air conditioning, etc. – and the service that these loads provide for the<br />
company – quantity <strong>of</strong> products manufactured, comfort <strong>of</strong> visitors to a supermarket,<br />
ambient temperature in a refrigerated room, etc.<br />
The information system is then ready to be used on a day to day basis by users to<br />
achieve energy efficiency objectives set by senior managers in the company.<br />
4.1 Physical value acquisition<br />
The quality <strong>of</strong> data starts with the measurement itself: at the right place, the right<br />
time and just the right amount.<br />
Basically, <strong>electrical</strong> measurement is based on voltage and current going through the<br />
conductors. These values lead to all the others: power, energy, power factor, etc.<br />
Firstly we will ensure consistency <strong>of</strong> the precision class <strong>of</strong> current transformers,<br />
voltage transformers and the precision <strong>of</strong> the measurement devices themselves. The<br />
precision class will be lower for higher voltages: an error in the measurement <strong>of</strong> high<br />
voltage for example represents a very large amount <strong>of</strong> energy.<br />
The total error is the quadratic sum <strong>of</strong> each error.<br />
2 2 2<br />
∑<strong>of</strong><br />
error = error + error + ... + error<br />
Example:<br />
a device with an error <strong>of</strong> 2% connected on a CT ’s with an error <strong>of</strong> 2% that means:<br />
2 2<br />
∑<strong>of</strong><br />
error = ( 2) + ( 2)<br />
= 2,828%<br />
That could mean a loss <strong>of</strong> 2 828 kWh for 100 000 kWh <strong>of</strong> consumption.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Communication<br />
(information to<br />
understanding)<br />
Data collection<br />
Action<br />
(understanding<br />
to results)
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
A CT is defined by:<br />
b its transformation ratio. For example: 50/5A<br />
b precision class Cl. Example: Cl=0.5 generally<br />
b precision power in VA to supply power to<br />
the measurement devices on the secondary.<br />
Example: 1.25 VA<br />
b limit precision factor indicated as a factor<br />
applied to In before saturation.<br />
Example: FLP (or Fs) =10 for measurement<br />
devices with a precision power that is in<br />
conformity.<br />
PM700 measurement unit<br />
4 From <strong>electrical</strong> measurement to<br />
<strong>electrical</strong> information<br />
Voltage measurement<br />
In low voltage, the voltage measurement is directly made by the measurement<br />
device. When the voltage level becomes incompatible with the device capacity, for<br />
example in medium voltage, we have to put in voltage transformers.<br />
A VT (Voltage transformer) is defined by:<br />
b its primary voltage and secondary voltage<br />
b its apparent power<br />
b its precision class<br />
Current measurement<br />
Current measurement is made by split or closed-core CT’s placed around the phase<br />
and neutral conductors as appropriate.<br />
According to the required precision for measurement, the CT used for the protection<br />
relay also allows current measurement under normal conditions.<br />
In particular, to measure energy, we consider two objectives:<br />
b A contractual billing objective, e.g. between an electricity company and its client<br />
or even between an airport manager (sub-billing) and stores renting airport surface<br />
areas. In this case IEC 62053-21 for Classes 1 and 2 and IEC 62053-22 for Classes<br />
0.5S and 0.2S become applicable to measure active energy.<br />
The full measurement chain – CT, VT and measurement unit – can reach a precision<br />
class Cl <strong>of</strong> 1 in low voltage, Cl 0.5 in medium voltage and 0.2 in high voltage, or even<br />
0.1 in the future.<br />
b An internal cost allocation objective for the company, e.g. to break-down the<br />
cost <strong>of</strong> electricity for each product produced in a specific workshop. In this case <strong>of</strong><br />
a precision class between 1 and 2 for the whole chain (CT, VT and measurement<br />
station) is sufficient.<br />
It is recommended to match the full measurement chain precision with actual<br />
measurement requirements: there is no one single universal solution, but a good<br />
technical and economic compromise according to the requirement to be satisfied.<br />
Note that the measurement precision also has a cost, to be compared with the return<br />
on investment that we are expecting.<br />
<strong>General</strong>ly gains in terms <strong>of</strong> energy efficiency are even greater when the <strong>electrical</strong><br />
network has not been equipped in this way until this point. In addition, permanent<br />
modifications <strong>of</strong> the <strong>electrical</strong> network, according to the company’s activity, mainly<br />
cause us to search for significant and immediate optimizations straight away.<br />
Example:<br />
A class 1 analogue ammeter, rated 100 A, will display a measurement <strong>of</strong> +/-1 A<br />
at 100 A. However if it displays 2 A, the measurement is correct to within 1 A and<br />
therefore there is uncertainty <strong>of</strong> 50%.<br />
A class 1 energy measurement station such as PM710 Merlin Gerin – like all other<br />
Merlin Gerin Power Meter and Circuit Monitor Measurement Units – is accurate to<br />
1% throughout the measurement range as described in IEC standards 62053.<br />
Other physical measurements considerably enhance the data:<br />
b on/<strong>of</strong>f, open/closed operating position <strong>of</strong> devices, etc.<br />
b energy metering impulse<br />
b transformer, motor temperature<br />
b hours operation, quantity <strong>of</strong> switching operations<br />
b motor load<br />
b UPS battery load<br />
b event logged equipment failures<br />
b etc<br />
4.2 Electrical data for real objectives<br />
Electrical data is transformed into information that is usually intended to satisfy<br />
several objectives:<br />
b It can modify the behaviour <strong>of</strong> users to manage energy wisely and finally lowers<br />
overall energy costs.<br />
b It can contribute to field staff efficiency increase<br />
b It can contribute to decrease the cost <strong>of</strong> Energy<br />
b It can contribute to save energy by understanding how it is used and how assets<br />
and process can be optimized to be more energy efficient<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
K11<br />
© Schneider Electric - all rights reserved
K12<br />
© Schneider Electric - all rights reserved<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Fig. K9 : Facility utility costs parallel the visualisation <strong>of</strong> an<br />
iceberg<br />
4 From <strong>electrical</strong> measurement to<br />
<strong>electrical</strong> information<br />
b It may help in optimizing and increasing the life duration <strong>of</strong> the assets associated to<br />
the <strong>electrical</strong> network<br />
b And finally it may be a master piece in increasing the productivity <strong>of</strong> the associated<br />
process (industrial process or even <strong>of</strong>fice, building management), by preventing, or<br />
reducing downtime, or insuring higher quality energy to the loads.<br />
Facility utility costs parallel the visualization <strong>of</strong> an iceberg (Fig. K9). While an iceberg<br />
seems large above the surface, the size is completely overwhelming beneath the<br />
surface. Similarly, <strong>electrical</strong> bills are brought to the surface each month when your<br />
power provider sends you a bill. Savings in this area are important and can be<br />
considerable enough to be the only justification needed for a power monitoring<br />
system. However, there are other less obvious yet more significant savings<br />
opportunities to be found below the surface if you have the right tools at your<br />
disposal.<br />
Modify the behaviour <strong>of</strong> energy users<br />
Using cost allocation reports, you can verify utility billing accuracy, distribute bills<br />
internally by department, make effective fact-based energy decisions and drive<br />
accountability in every level <strong>of</strong> your organization. Then providing ownership <strong>of</strong><br />
electricity costs to the appropriate level in an organization, you modify the behaviour<br />
<strong>of</strong> users to manage energy wisely and finally lowers overall energy costs.<br />
Increase field staff efficiency<br />
One <strong>of</strong> the big challenges <strong>of</strong> field staff in charge <strong>of</strong> the <strong>electrical</strong> network is to make<br />
the right decision and operate in the minimum time.<br />
The first need <strong>of</strong> such people is then to better know what happens on the network,<br />
and possibly to be informed everywhere on the concerned site.<br />
This site-wise transparency is a key feature that enables a field staff to:<br />
b Understand the <strong>electrical</strong> energy flows – check that the network is correctly set-up,<br />
balanced, what are the main consumers, at what period <strong>of</strong> the day, or the week…<br />
b Understand the network behaviour – a trip on a feeder is easier to understand<br />
when you have access to information from downstream loads.<br />
b Be spontaneously informed on events, even outside the concerned site by using<br />
today’s mobile communication<br />
b Going straight forward to the right location on the site with the right spare part, and<br />
with the understanding <strong>of</strong> the complete picture<br />
b Initiate a maintenance action taking into account the real usage <strong>of</strong> a device, not too<br />
early and not too late<br />
b Therefore, providing to the electrician a way to monitor the <strong>electrical</strong> network can<br />
appear as a powerful mean to optimize and in certain case drastically reduce the<br />
cost <strong>of</strong> power.<br />
Here are some examples <strong>of</strong> the main usage <strong>of</strong> the simplest monitoring systems:<br />
b Benchmark between zones to detect abnormal consumption.<br />
b Track unexpected consumption.<br />
b Ensure that power consumption is not higher that your competitors.<br />
b Choose the right Power delivery contract with the Power Utility.<br />
b Set-up simple load-shedding just focusing on optimizing manageable loads such<br />
as lights.<br />
b Be in a position to ask for damage compensation due to non-quality delivery<br />
from the Power Utilities – The process has been stopped because <strong>of</strong> a sag on the<br />
network.<br />
Implementing energy efficiency projects<br />
The Power monitoring system will deliver information that support a complete<br />
energy audit <strong>of</strong> a factility. Such audit can be the way to cover not only electricity<br />
but also Water, Air, Gas and Steam. Measures, benchmark and normalized energy<br />
consumption information will tell how efficient the industrial facilities and process<br />
are. Appropriate action plans can then be put in place. Their scope can be as wide<br />
as setting up control lighting, Building automation systems, variable speed drive,<br />
process automation, etc.<br />
Optimizing the assets<br />
One increasing fact is that <strong>electrical</strong> network evolves more and more and then a<br />
recurrent question occurs : Will my network support this new evolution?<br />
This is typically where a Monitoring system can help the network owner in making<br />
the right decision.<br />
By its logging activity, it can archive the real use <strong>of</strong> the assets and then evaluate<br />
quite accurately the spare capacity <strong>of</strong> a network, or a switchboard, a transformer…<br />
A better use <strong>of</strong> an asset may increase its life duration.<br />
Monitoring systems can provide accurate information <strong>of</strong> the exact use <strong>of</strong> an asset<br />
and then the maintenance team can decide the appropriate maintenance operation,<br />
not too late, or not too early.<br />
In some cases also, the monitoring <strong>of</strong> harmonics can be a positive factor for the life<br />
duration <strong>of</strong> some assets (such as motors or transformers).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Micrologic tripping unit for<br />
Masterpact<br />
TeSys U motor controller<br />
4 From <strong>electrical</strong> measurement to<br />
<strong>electrical</strong> information<br />
Increasing the productivity by reducing the downtime<br />
Downtime is the nightmare <strong>of</strong> any people in charge <strong>of</strong> an <strong>electrical</strong> network. It may<br />
cause dramatic loss for the company, and the pressure for powering up again in the<br />
minimum time – and the associated stress for the operator – is very high.<br />
A monitoring and control system can help reducing the downtime very efficiently.<br />
Without speaking <strong>of</strong> a remote control system which are the most sophisticated<br />
system and which may be necessary for the most demanding application, a simple<br />
monitoring system can already provide relevant information that will highly contribute<br />
in reducing the downtime:<br />
b Making the operator spontaneously informed, even remote, even out <strong>of</strong> the<br />
concerned site (Using the mobile communication such as DECT network or GSM/<br />
SMS)<br />
b Providing a global view <strong>of</strong> the whole network status<br />
b Helping the identification <strong>of</strong> the faulty zone<br />
b Having remotely the detailed information attached to each event caught by the field<br />
devices (reason for trip for example)<br />
Then remote control <strong>of</strong> a device is a must but not necessary mandatory. In many<br />
cases, a visit <strong>of</strong> the faulty zone is necessary where local actions are possible.<br />
Increasing the productivity by improving the Energy Quality<br />
Some loads can be very sensitive to electricity quality, and operators may face<br />
unexpected situations if the Energy quality is not under control.<br />
Monitoring the Energy quality is then an appropriate way to prevent such event and /<br />
or to fix specific issue.<br />
4.3 Measurement starts with the “stand alone<br />
product” solution<br />
The choice <strong>of</strong> measurement products in <strong>electrical</strong> equipment is made according to<br />
your energy efficiency priorities and also current technological advances:<br />
b measurement and protection functions <strong>of</strong> the LV or MV <strong>electrical</strong> network are<br />
integrated in the same device,<br />
Example: Sepam metering and protection relays, Micrologic tripping unit for<br />
Masterpact, TeSys U motor controller, NRC12 capacitor bank controller, Galaxy<br />
UPSs<br />
b the measurement function is in the device, separate from the protection function,<br />
e.g. built on board the LV circuit breaker.<br />
Example: PowerLogic Circuit Monitor high performance metering unit<br />
The progress made in real time industrial electronics and IT are used in a single<br />
device:<br />
b to meet requirements for simplification <strong>of</strong> switchboards<br />
b to reduce acquisition costs and reduce the number <strong>of</strong> devices<br />
b to facilitate product developments by s<strong>of</strong>tware upgrade procedures<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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K14<br />
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K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Examples Power Meter, Circuit<br />
Monitor<br />
Keep control over power consumption<br />
Measurement units MV protection and<br />
measurement relays<br />
Below we give examples <strong>of</strong> measurements available via Modbus, RS485 or Ethernet<br />
Fig. K10:<br />
LV protection and<br />
measurement relays<br />
SEPAM Masterpact &<br />
Compact Micrologic<br />
trip units<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Capacitor bank<br />
regulators<br />
Insulation monitors<br />
Varlogic Vigilohm System<br />
Power, inst., max., min. b b b b -<br />
Energy, reset capability b b b - -<br />
Power factor, inst. b b b - -<br />
Cos φ inst. - - - b -<br />
Improve power supply availability<br />
Current, inst., max., min., unbalance b b b b -<br />
Current, wave form capture b b b - -<br />
Voltage, inst., max., min., unbalance b b b b -<br />
Voltage, wave form capture b b b - -<br />
Device status b b b b -<br />
Faults history b b b - -<br />
Frequency, inst., max., min. b b b - -<br />
THDu, THDi b b b b -<br />
Manage <strong>electrical</strong> <strong>installation</strong> better<br />
Load temperature, load and device<br />
thermal state<br />
b b - b -<br />
Insulating resistance - - - - b<br />
Motor controllers LV speeddrives LV s<strong>of</strong>tstarters MV s<strong>of</strong>tstarters UPSs<br />
Examples TeSys U ATV.1 ATS.8 Motorpact RVSS Galaxy<br />
Keep control over power consumption<br />
Power, inst., max., min. - b - b b<br />
Energy, reset capability - b b b -<br />
Power factor, inst. - - b b b<br />
Improve power supply availability<br />
Current, inst., max., min., unbalance b b b b b<br />
Current, wave form capture - - - b b<br />
Device status b b b b b<br />
Faults history b b b b -<br />
THDu, THDi - b - - -<br />
Manage <strong>electrical</strong> <strong>installation</strong> better<br />
Load temperature, load and device<br />
thermal state<br />
b b b b b<br />
Motor running hours - b b b -<br />
Battery follow up - - - - b<br />
Fig. K10 : Examples <strong>of</strong> measurements available via Modbus, RS485 or Ethernet<br />
4 From <strong>electrical</strong> measurement to<br />
<strong>electrical</strong> information
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Fig. K11 : Example <strong>of</strong> <strong>electrical</strong> network protected and<br />
monitored via the Intranet site<br />
Fig. K12 : A Test to stop all lighting B Test to stop air conditioning<br />
Here consumption during non-working hours seems excessive, consequently two decisions were taken:<br />
b reducing night time lighting<br />
b stopping air conditioning during weekends<br />
The new curve obtained shows a significant drop in consumption.<br />
4 From <strong>electrical</strong> measurement to<br />
<strong>electrical</strong> information<br />
Example <strong>of</strong> solutions for a medium-sized site:<br />
Analysesample Ltd. is a company specialized in analyzing industrial samples from<br />
regional factories: metals, plastics, etc., to certify their chemical characteristics.<br />
The company wants to carry out better control <strong>of</strong> its <strong>electrical</strong> consumption for the<br />
existing <strong>electrical</strong> furnaces, its air conditioning system and to ensure quality <strong>of</strong><br />
<strong>electrical</strong> supply for high-precision electronic devices used to analyze the samples.<br />
Electrical network protected and monitored via the Intranet site<br />
The solution implemented involves recovering power data via metering units that<br />
also allows measurement <strong>of</strong> basic <strong>electrical</strong> parameters as well as verification <strong>of</strong><br />
energy power quality. Connected to a web server, an Internet browser allows to use<br />
them very simply and export data in a Micros<strong>of</strong>t Excel type spreadsheet. Power<br />
curves can be plotted in real time by the spreadsheet (Fig. K11).<br />
Therefore no IT investment, either in s<strong>of</strong>tware or hardware, is necessary to use the<br />
data.<br />
For example to reduce the electricity bill and limit consumption during nighttime<br />
and weekends, we have to study trend curves supplied by the measurement units<br />
(Fig. K12).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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K16<br />
© Schneider Electric - all rights reserved<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
5 Communication and Information<br />
System<br />
Most organisations will already have some level <strong>of</strong> energy information system, even<br />
if it is not identified or managed as one. It should be appreciated that in a changing<br />
working world, any information system will need to develop to meet its prime<br />
objective - supporting management decision making: a key point is to make the<br />
energy information visible at any level <strong>of</strong> the organization through the communication<br />
infrastructure.<br />
Energy data is important data, it is one <strong>of</strong> the company’s assets. The company has<br />
IT managers who are already in charge <strong>of</strong> managing its other IT systems. These<br />
are important players in the power monitoring system and above all in that for data<br />
exchange within the corporate organization.<br />
5.1 Communication network at product, equipment<br />
and site level<br />
The day-to-day working <strong>of</strong> the energy information system can be illustrated by a<br />
closed loop diagram (Fig. K13).<br />
Modbus*<br />
Data<br />
* Communication network<br />
Fig. K13 : System hierarchy<br />
Various resources are used to send data from metering and protection devices<br />
installed in the user’s <strong>electrical</strong> cabinets, e.g. via Schneider ElectricTransparent<br />
Ready.<br />
The Modbus communication protocol<br />
Modbus is an industrial messaging protocol between equipment that is<br />
interconnected via a physical transmission link e.g. RS 485 or Ethernet (via TCP/IP)<br />
or modem (GSM, Radio etc). This protocol is very widely implemented on metering<br />
and protection products for <strong>electrical</strong> networks.<br />
Initially created by Schneider Electric, Modbus is now a public resource managed<br />
by an independent organization Modbus-IDA – enabling total opening up <strong>of</strong> its<br />
specification. An industrial standard since 1979, Modbus allows millions <strong>of</strong> products<br />
to communicate with one another.<br />
The IETF, international authority managing the Internet, has approved the creation<br />
<strong>of</strong> a port (502) for products connected to the Internet/Intranet and using the Ethernet<br />
Modbus TCP/IP communication protocol.<br />
Modbus is a query/reply process between two pieces <strong>of</strong> equipment based on data<br />
reading and writing services (function codes).<br />
The query is emitted by a single “master”, the reply is sent only by the “slave”<br />
equipment identified in the query (Fig. K14).<br />
Each “slave” product connected to the Modbus network is set by the user with an ID<br />
number, called the Modbus address, between 1 and 247.<br />
The “master” – for example a web server included in an <strong>electrical</strong> cabinet<br />
– simultaneously queries all <strong>of</strong> the products with a message comprising its target’s<br />
address, function code, memory location in the product and quantity <strong>of</strong> information,<br />
at most 253 octets.<br />
Only a product set with the corresponding address answers the request for data.<br />
Exchange is only carried out on the initiative <strong>of</strong> the master (here the web server): this<br />
is the master-slave Modbus operating procedure.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Intranet*<br />
Information<br />
Communicating<br />
measurement device*<br />
Energy information systems<br />
Understanding
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
5 Communication and Information<br />
System<br />
This query procedure followed by a reply, implies that the master will have all <strong>of</strong> the<br />
data available in a product when it is queried.<br />
The “master” manages all <strong>of</strong> the transaction queries successively if they are intended<br />
for the same product. This arrangement leads to the calculation <strong>of</strong> a maximum<br />
number <strong>of</strong> products connected to the master to optimize an acceptable response<br />
time for the query initiator, particularly when it is a low rate RS485 link.<br />
Fig. K14 : The function codes allow writing or reading <strong>of</strong> data.<br />
A transmission error s<strong>of</strong>tware detection mechanism called CRC16 allows a message with an<br />
error to be repeated and only the product concerned to respond.<br />
Your Intranet network<br />
Data exchange from industrial data basically uses web technologies implemented<br />
permanently on the corporate communication network, and more particularly on its<br />
Intranet.<br />
The IT infrastructure manages the cohabitation <strong>of</strong> s<strong>of</strong>tware applications: the<br />
company uses it to operate applications for the <strong>of</strong>fice, printing, data backup, for the<br />
corporate IT system, accounting, purchasing, ERP, production facility control, API,<br />
MES, etc. The cohabitation <strong>of</strong> data on the same communication network does not<br />
pose any particular technological problem.<br />
When several PC’s, printers and servers are connected to one another in the<br />
company’s buildings, very probably using the Ethernet local network and web<br />
services: this company is then immediately eligible to have energy efficiency data<br />
delivered by its <strong>electrical</strong> cabinets. Without any s<strong>of</strong>tware development, all they need<br />
is an Micros<strong>of</strong>t Internet Explorer type Internet browser.<br />
The data from these applications cross the local broadband Ethernet network up to<br />
1 Gb/s: the communication media generally used in this world is copper or optic fiber,<br />
which allows connection everywhere, in commercial or industrial buildings and in<br />
<strong>electrical</strong> premises.<br />
If the company also has an internal Intranet communication network for emailing<br />
and sharing web servers data, it uses an extremely common standardized<br />
communication protocol: TCP/IP.<br />
The TCP/IP communication protocol is <strong>design</strong>ed for widely used web services such<br />
as HTTP to access web pages, SMTP for electronic messaging between other<br />
services.<br />
Applications SNMP NTP RTPS DHCP TFTP FTP HTTP SMTP Modbus<br />
Transport UDP TCP<br />
Link IP<br />
Physical Ethernet 802.3 and Ethernet II<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
K17<br />
© Schneider Electric - all rights reserved
K18<br />
© Schneider Electric - all rights reserved<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
5 Communication and Information<br />
System<br />
Electrical data recorded in industrial web servers installed in <strong>electrical</strong> cabinets are<br />
sent using the same standardized TCP/IP protocol in order to limit the recurrent IT<br />
maintenance costs that are intrinsic in an IT network. This is the operating principle<br />
<strong>of</strong> Schneider Electric Transparent Ready TM for communication <strong>of</strong> data on energy<br />
efficiency. The <strong>electrical</strong> cabinet is autonomous without the need for any additional<br />
IT system on a PC, all <strong>of</strong> the data related to energy efficiency is recorded and can be<br />
circulated in the usual way via the intranet, GSM, fixed telephone link, etc.<br />
Security<br />
Employees that are well informed, more efficient and working in complete <strong>electrical</strong><br />
safety: they no longer need to go into <strong>electrical</strong> rooms or make standard checks<br />
on <strong>electrical</strong> devices - they just have to consult data. Under these conditions,<br />
communicative systems give the company’s employees immediate and significant<br />
gains and avoid worrying about making mistakes.<br />
It becomes possible for electricians, maintenance or production technicians, on-site<br />
or visiting managers to work together in complete safety.<br />
According to the sensitivity <strong>of</strong> data, the IT manager will simply give users the<br />
appropriate access rights.<br />
Marginal impact on local network maintenance<br />
The company’s IT manager has technical resources to add and monitor equipment<br />
to the local company network.<br />
Based on standard web services including the Modbus protocol on TCP/IP, and<br />
due to the low level <strong>of</strong> bandwidth requirement characteristic in <strong>electrical</strong> network<br />
monitoring systems as well as the use <strong>of</strong> technologies that are not impacted by<br />
viruses and worldwide IT standards, the IT manager does not have to make any<br />
specific investment to preserve the local network performance level or to protect<br />
against any additional security problems (virus, hacking, etc.).<br />
Empowering external partners<br />
According to the company’s security policy, it becomes possible to use support<br />
services <strong>of</strong> the usual partners in the <strong>electrical</strong> sector: contractors, utilities managers,<br />
panelbuilders, systems integrators or Schneider Electric Services can provide<br />
remote assistance and <strong>electrical</strong> data analysis to the company consuming electricity.<br />
The messaging web service can regularly send data by email or web pages can be<br />
remotely consulted using the appropriate techniques.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
Function<br />
levels<br />
<strong>General</strong><br />
purpose<br />
site<br />
monitoring<br />
Specialised<br />
network<br />
monitoring<br />
Basic<br />
monitoring<br />
1<br />
Eqt server<br />
Intelligent<br />
Power<br />
Equipment<br />
Web<br />
browser<br />
standard<br />
Other<br />
utilities<br />
Fig. K15 : Monitoring system positioning<br />
2<br />
Eqt server<br />
Power<br />
Equipment<br />
5 Communication and Information<br />
System<br />
5.2 From Network Monitoring and Control System to<br />
Intelligent Power Equipment<br />
Traditionally and for years, monitoring and control systems have been centralized<br />
and based on SCADA (Supervisory, Control and Data acquisition) automation<br />
systems.<br />
Deciding on investing in such system – noted (3) in Figure K15 – was really<br />
reserved for high demanding <strong>installation</strong>, because either they were big power<br />
consumers, or their process was very sensitive to Power non quality.<br />
Based on automation technology, such systems were very <strong>of</strong>ten <strong>design</strong>ed,<br />
customised by a system integrator, and then delivered on site. However the initial<br />
cost, the skills needed to correctly operate such system, and the cost <strong>of</strong> upgrades to<br />
follow the evolutions <strong>of</strong> the network may have discouraged potential users to invest.<br />
Then based on a dedicated solution for electrician, the other approach noted (2)<br />
is much more fitting the <strong>electrical</strong> network specific needs and really increases the<br />
payback <strong>of</strong> such system. However, due to its centralised architecture, the level cost<br />
<strong>of</strong> such solution may still appear high.<br />
On some sites Type (2) and (3) can cohabit, providing the most accurate information<br />
to the electrician when needed.<br />
Nowadays, a new concept <strong>of</strong> intelligent Power equipment – noted (1) – has come.<br />
considered as an entering step for going to level 2 or 3, due the ability <strong>of</strong> these<br />
solutions to co-exist on a site.<br />
Specialised<br />
monitoring<br />
such as<br />
Power Logic<br />
SMS<br />
Eqt gateway<br />
Power<br />
Equipment<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
3<br />
Other<br />
utilities Process<br />
Standard network Sensitive <strong>electrical</strong> networks High demanding sites<br />
<strong>General</strong><br />
purpose<br />
monitoring<br />
system<br />
System<br />
complexity<br />
K19<br />
© Schneider Electric - all rights reserved
K20<br />
© Schneider Electric - all rights reserved<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
5 Communication and Information<br />
System<br />
Intelligent equipment based architecture (see Fig. K16)<br />
This new architecture has appeared recently due to Web technology capabilities, and<br />
can really be positioned as an entry point into monitoring systems.<br />
Based on Web technologies it takes the maximum benefits <strong>of</strong> standard<br />
communication services and protocols, and license-free s<strong>of</strong>tware.<br />
The access to electricity information can be done from everywhere in the site, and<br />
<strong>electrical</strong> staff can gain a lot in efficiency.<br />
Openness to the Internet is also <strong>of</strong>fered for out <strong>of</strong> the site services.<br />
Standard remote<br />
Web browser<br />
Internet<br />
Equipment server<br />
Gateway<br />
Electrician specialized centralised architecture (see Fig. K17)<br />
Dedicated to electrician, this architecture is based on a specific supervision<br />
centralised mean that fully match the needs for monitoring an <strong>electrical</strong> network.<br />
Then it <strong>of</strong>fers naturally a lower level <strong>of</strong> skill to set up and maintain it – all Electrical<br />
Distribution devices are already present in a dedicated library. Finally its purchase<br />
cost is really minimized, due the low level <strong>of</strong> system integrator effort.<br />
Fig. K17 : ED specialist monitoring system<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
1 2 3<br />
Circuit breakers<br />
Fig. K16 : Intelligent equipment architecture<br />
Gateway<br />
1 2 3<br />
Circuit breakers<br />
Modbus<br />
Standard local<br />
Web browser<br />
Intranet (Ethernet/IP)<br />
Modbus<br />
Meter 1<br />
Dedicated supervisor<br />
for electrician<br />
Modbus (SL or Ethernet/IP)<br />
Intelligence Power Equipment<br />
Meter 2 Meter 3<br />
Communicating Power Equipment<br />
Meter 1 Meter 2 Meter 3
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
5 Communication and Information<br />
System<br />
Conventional general purpose centralised architecture (see Fig. K18)<br />
Here is a typical architecture based on standard automation pieces such as SCADA<br />
systems, and gateways.<br />
Despite its real efficiency, such architecture suffered from some drawbacks such as :<br />
b The level <strong>of</strong> skills needed to operate it<br />
b The poor upgradability<br />
b And at the end the risky payback <strong>of</strong> such solutions<br />
They have however no equivalent for high demanding sites, and appears very<br />
relevant for central operation rooms.<br />
Gateway<br />
1 2 3<br />
Circuit breakers<br />
Fig. K18 : Real-time conventional monitoring and control system<br />
5.3 e-Support becomes accessible<br />
The setting up <strong>of</strong> an information system to support a global energy efficiency<br />
approach very quickly leads to economic gains, in general with an ROI <strong>of</strong> less than 2<br />
years for electricity.<br />
An additional benefit, that is still underestimated today, is the leverage that this leads<br />
to in terms <strong>of</strong> information technologies in the <strong>electrical</strong> sector. The <strong>electrical</strong> network<br />
can be analyzed from time to time by third parties – in particular using external<br />
competencies via the internet for very specific issues:<br />
b Electricity supply contracts. Changing <strong>of</strong> supplier at a given point in time, e.g.<br />
permanent economic analysis <strong>of</strong> the costs related to consumption becomes possible<br />
without having to wait for an annual review.<br />
b Total management <strong>of</strong> <strong>electrical</strong> data – via internet – to transform it into relevant<br />
information that is fed back via a personalized web portal. Consumer usage<br />
information is now a value-added commodity, available to a wide range <strong>of</strong> users. It's<br />
easy to post customer usage data on the Internet – making it useful to the users is<br />
another matter.<br />
b Complex <strong>electrical</strong> fault diagnosis to call in an electrotechnical expert, a rare<br />
resource that is easily accessible on the web.<br />
b Monitoring <strong>of</strong> consumption and generating alerts in the case <strong>of</strong> abnormal<br />
consumption peaks.<br />
b A maintenance service that is no more than necessary to meet pressure on<br />
overheads via facility management services.<br />
Energy efficiency is no longer an issue that the company has to face on its own,<br />
many e-partners can back up the approach as necessary – in particular when the<br />
measurement and decision making assistance stage is reached, on condition that<br />
the <strong>electrical</strong> network is metered and communicative via internet.<br />
Implementation can be gradual starting by making a few key pieces <strong>of</strong> equipment<br />
communicative and gradually extending the system so as to be more accurate or to<br />
give wider coverage <strong>of</strong> the <strong>installation</strong>.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Modbus<br />
Conventional<br />
supervisor<br />
Modbus (SL or Ethernet/IP)<br />
Communicating Power Equipment<br />
Meter 1 Meter 2 Meter 3<br />
K21<br />
© Schneider Electric - all rights reserved
K22<br />
© Schneider Electric - all rights reserved<br />
K - Energy Efficiency in <strong>electrical</strong> distribution<br />
5 Communication and Information<br />
System<br />
The company can choose its policy: ask one or more partners to analyze the data,<br />
do it itself or combine these options.<br />
The company may decide to manage its <strong>electrical</strong> energy itself, or ask a partner to<br />
monitor the quality to ensure active monitoring <strong>of</strong> performances in terms <strong>of</strong> aging.<br />
Example:<br />
Schneider Electric proposes e-Services that <strong>of</strong>fers load data visualization and analysis<br />
application in ASP mode. It simplifies processes for tenants with geographically diverse locations<br />
by providing convenient integrated billing and usage information for all locations combined.<br />
The system turns customer usage data into useful information, easily accessible to all internal<br />
users. It helps control costs by showing customers how their organizations use power.<br />
A wide range <strong>of</strong> functionality serves the needs <strong>of</strong> staff from the same platform:<br />
Data Access and Analysis , Historical and Estimated Bills, Rate Comparison, What-if Analysis<br />
- Assess the impact <strong>of</strong> operational changes, such as shifting energy between time periods or<br />
reducing usage by fixed amounts or percentages, Automatic Alarming, Memorized Reports,<br />
Benchmarking - Benchmark usage data from multiple facilities by applying normalization factors<br />
such as square footage, operating hours, and units <strong>of</strong> production. Multiple Commodities - Access<br />
usage data for gas and water as well as electricity etc.<br />
New York Chicago Los Angeles Seattle<br />
Ethernet/VPN Ethernet/VPN<br />
Reports<br />
Fig. K19 : Typical solution exemple<br />
Electricity<br />
Water & Gas<br />
Power Quality<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
WEB<br />
Energy Cost<br />
Analysis<br />
Normalize data using:<br />
- Temperature<br />
- Occupancy rates<br />
- Rooms<br />
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<strong>Chapter</strong> L<br />
Power factor correction and<br />
harmonic filtering<br />
Contents<br />
Reactive energy and power factor L2<br />
1.1 The nature <strong>of</strong> reactive energy L2<br />
1.2 Equipment and appliances requiring reactive energy L2<br />
1.3 The power factor L3<br />
1.4 Practical values <strong>of</strong> power factor L4<br />
Why to improve the power factor? L5<br />
2.1 Reduction in the cost <strong>of</strong> electricity L5<br />
2.2 Technical/economic optimization L5<br />
How to improve the power factor? L7<br />
3.1 Theoretical principles L7<br />
3.2 By using what equipment? L7<br />
3.3 The choice between a fixed or automatically-regulated bank L9<br />
<strong>of</strong> capacitors<br />
Where to install power factor correction capacitors? L 0<br />
4.1 Global compensation L10<br />
4.2 Compensation by sector L10<br />
4.3 Individual compensation L11<br />
How to decide the optimum level <strong>of</strong> compensation? L 2<br />
5.1 <strong>General</strong> method L12<br />
5.2 Simplified method L12<br />
5.3 Method based on the avoidance <strong>of</strong> tariff penalties L14<br />
5.4 Method based on reduction <strong>of</strong> declared maximum apparent<br />
power (kVA) L14<br />
Compensation at the terminals <strong>of</strong> a transformer L 5<br />
6.1 Compensation to increase the available active power output L15<br />
6.2 Compensation <strong>of</strong> reactive energy absorbed by the transformer L16<br />
Power factor correction <strong>of</strong> induction motors L 8<br />
7.1 Connection <strong>of</strong> a capacitor bank and protection settings L18<br />
7.2 How self-excitation <strong>of</strong> an induction motor can be avoided L19<br />
Example <strong>of</strong> an <strong>installation</strong> before and L20<br />
after power-factor correction<br />
The effects <strong>of</strong> harmonics L2<br />
9.1 Problems arising from power-system harmonics L21<br />
9.2 Possible solutions L21<br />
9.3 Choosing the optimum solution L23<br />
Implementation <strong>of</strong> capacitor banks L24<br />
10.1 Capacitor elements L24<br />
10.2 Choice <strong>of</strong> protection, control devices and connecting cables L25<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
L<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
L2<br />
L - Power factor correction and<br />
harmonic filtering<br />
Alternating current systems supply two forms <strong>of</strong><br />
energy:<br />
b “Active” energy measured in kilowatt hours<br />
(kWh) which is converted into mechanical work,<br />
heat, light, etc<br />
b “Reactive” energy, which again takes two<br />
forms:<br />
v “Reactive” energy required by inductive<br />
circuits (transformers, motors, etc.),<br />
v “Reactive” energy supplied by capacitive<br />
circuits (cable capacitance, power capacitors,<br />
etc)<br />
Fig. L2 : Power consuming items that also require reactive<br />
energy<br />
Reactive energy and power<br />
factor<br />
. The nature <strong>of</strong> reactive energy<br />
All inductive (i.e. electromagnetic) machines and devices that operate on AC systems<br />
convert <strong>electrical</strong> energy from the power system generators into mechanical work<br />
and heat. This energy is measured by kWh meters, and is referred to as “active”<br />
or “wattful” energy. In order to perform this conversion, magnetic fields have to be<br />
established in the machines, and these fields are associated with another form <strong>of</strong><br />
energy to be supplied from the power system, known as “reactive” or “wattless”<br />
energy.<br />
The reason for this is that inductive circuit cyclically absorbs energy from the system<br />
(during the build-up <strong>of</strong> the magnetic fields) and re-injects that energy into the system<br />
(during the collapse <strong>of</strong> the magnetic fields) twice in every power-frequency cycle.<br />
An exactly similar phenomenon occurs with shunt capacitive elements in a power<br />
system, such as cable capacitance or banks <strong>of</strong> power capacitors, etc. In this case,<br />
energy is stored electrostatically. The cyclic charging and discharging <strong>of</strong> capacitive<br />
circuit reacts on the generators <strong>of</strong> the system in the same manner as that described<br />
above for inductive circuit, but the current flow to and from capacitive circuit in exact<br />
phase opposition to that <strong>of</strong> the inductive circuit. This feature is the basis on which<br />
power factor correction schemes depend.<br />
It should be noted that while this “wattless” current (more accurately, the “wattless”<br />
component <strong>of</strong> a load current) does not draw power from the system, it does cause<br />
power losses in transmission and distribution systems by heating the conductors.<br />
In practical power systems, “wattless” components <strong>of</strong> load currents are invariably<br />
inductive, while the impedances <strong>of</strong> transmission and distribution systems are<br />
predominantly inductively reactive. The combination <strong>of</strong> inductive current passing<br />
through an inductive reactance produces the worst possible conditions <strong>of</strong> voltage<br />
drop (i.e. in direct phase opposition to the system voltage).<br />
For these reasons (transmission power losses and voltage drop), the power-supply<br />
authorities reduce the amount <strong>of</strong> “wattless” (inductive) current as much as possible.<br />
“Wattless” (capacitive) currents have the reverse effect on voltage levels and produce<br />
voltage-rises in power systems.<br />
The power (kW) associated with “active” energy is usually represented by the letter P.<br />
The reactive power (kvar) is represented by Q. Inductively-reactive power is<br />
conventionally positive (+ Q) while capacitively-reactive power is shown as a<br />
negative quantity (- Q).<br />
The apparent power S (kVA) is a combination <strong>of</strong> P and Q (see Fig. L ).<br />
Sub-clause 1.3 shows the relationship between P, Q, and S.<br />
Q<br />
(kvar)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
S<br />
(kVA)<br />
P<br />
(kW)<br />
Fig. L1 : An electric motor requires active power P and reactive power Q from the power system<br />
.2 Equipement and appliances requiring reactive<br />
energy<br />
All AC equipement and appliances that include electromagnetic devices, or depend<br />
on magnetically-coupled windings, require some degree <strong>of</strong> reactive current to create<br />
magnetic flux.<br />
The most common items in this class are transformers and reactors, motors and<br />
discharge lamps (with magnetic ballasts) (see Fig. L2).<br />
The proportion <strong>of</strong> reactive power (kvar) with respect to active power (kW) when an<br />
item <strong>of</strong> equipement is fully loaded varies according to the item concerned being:<br />
b 65-75% for asynchronous motors<br />
b 5-10% for transformers
L - Power factor correction and<br />
harmonic filtering<br />
The power factor is the ratio <strong>of</strong> kW to kVA.<br />
The closer the power factor approaches its<br />
maximum possible value <strong>of</strong> 1, the greater the<br />
benefit to consumer and supplier.<br />
PF = P (kW) / S (kVA)<br />
P = Active power<br />
S = Apparent power<br />
Reactive energy and power<br />
factor<br />
.3 The power factor<br />
Definition <strong>of</strong> power factor<br />
The power factor <strong>of</strong> a load, which may be a single power-consuming item, or a<br />
number <strong>of</strong> items (for example an entire <strong>installation</strong>), is given by the ratio <strong>of</strong> P/S i.e.<br />
kW divided by kVA at any given moment.<br />
The value <strong>of</strong> a power factor will range from 0 to 1.<br />
If currents and voltages are perfectly sinusoidal signals, power factor equals cos ϕ.<br />
A power factor close to unity means that the reactive energy is small compared with<br />
the active energy, while a low value <strong>of</strong> power factor indicates the opposite condition.<br />
Power vector diagram<br />
b Active power P (in kW)<br />
v Single phase (1 phase and neutral): P = V I cos ϕ<br />
v Single phase (phase to phase): P = U I cos ϕ<br />
v Three phase (3 wires or 3 wires + neutral): P = 3U I cos ϕ<br />
b Reactive power Q (in kvar)<br />
v Single phase (1 phase and neutral): P = V I sin ϕ<br />
v Single phase (phase to phase): Q = U I sin ϕ<br />
v Three phase (3 wires or 3 wires + neutral): P = 3 U I sin ϕ<br />
b Apparent power S (in kVA)<br />
v Single phase (1 phase and neutral): S = V I<br />
v Single phase (phase to phase): S = U I<br />
v Three phase (3 wires or 3 wires + neutral): P = 3 U I<br />
where:<br />
V = Voltage between phase and neutral<br />
U = Voltage between phases<br />
I = Line current<br />
ϕ = Phase angle between vectors V and I.<br />
v For balanced and near-balanced loads on 4-wire systems<br />
Current and voltage vectors, and derivation <strong>of</strong> the power diagram<br />
The power “vector” diagram is a useful artifice, derived directly from the true rotating<br />
vector diagram <strong>of</strong> currents and voltage, as follows:<br />
The power-system voltages are taken as the reference quantities, and one phase<br />
only is considered on the assumption <strong>of</strong> balanced 3-phase loading.<br />
The reference phase voltage (V) is co-incident with the horizontal axis, and the<br />
current (I) <strong>of</strong> that phase will, for practically all power-system loads, lag the voltage by<br />
an angle ϕ.<br />
The component <strong>of</strong> I which is in phase with V is the “wattful” component <strong>of</strong> I and is<br />
equal to I cos ϕ, while VI cos ϕ equals the active power (in kW) in the circuit, if V is<br />
expressed in kV.<br />
The component <strong>of</strong> I which lags 90 degrees behind V is the wattless component <strong>of</strong><br />
I and is equal to I sin ϕ, while VI sin ϕ equals the reactive power (in kvar) in the<br />
circuit, if V is expressed in kV.<br />
If the vector I is multiplied by V, expressed in kV, then VI equals the apparent power<br />
(in kVA) for the circuit.<br />
The simple formula is obtained: S 2 = P 2 + Q 2<br />
The above kW, kvar and kVA values per phase, when multiplied by 3, can therefore<br />
conveniently represent the relationships <strong>of</strong> kVA, kW, kvar and power factor for a total<br />
3-phase load, as shown in Figure L3 .<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
ϕ<br />
Fig. L3 : Power diagram<br />
Q = VI sin ϕ (kvar)<br />
P = VI cos ϕ (kW)<br />
S = VI (kVA)<br />
V<br />
P = Active power<br />
Q = Reactive power<br />
S = Apparent power<br />
L3<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
L4<br />
L - Power factor correction and<br />
harmonic filtering<br />
ϕ<br />
P = 56 kW<br />
S = 65 kVA<br />
Fig. L5 : Calculation power diagram<br />
Q = 33 kvar<br />
Reactive energy and power<br />
factor<br />
An example <strong>of</strong> power calculations (see Fig. L4 )<br />
Type <strong>of</strong> Apparent power Active power Reactive power<br />
circuit S (kVA) P (kW) Q (kvar)<br />
Single-phase (phase and neutral) S = VI P = VI cos ϕ Q = VI sin ϕ<br />
Single-phase (phase to phase) S = UI P = UI cos ϕ Q = UI sin ϕ<br />
Example 5 kW <strong>of</strong> load 10 kVA 5 kW 8.7 kvar<br />
cos ϕ = 0.5<br />
Three phase 3-wires or 3-wires + neutral S = 3 UI P = 3 UI cos ϕ Q = 3 UI sin ϕ<br />
Example Motor Pn = 51 kW 65 kVA 56 kW 33 kvar<br />
cos ϕ = 0.86<br />
ρ = 0.91 (motor efficiency)<br />
Fig. L4 : Example in the calculation <strong>of</strong> active and reactive power<br />
.4 Practical values <strong>of</strong> power factor<br />
The calculations for the three-phase example above are as follows:<br />
Pn = delivered shaft power = 51 kW<br />
P = active power consumed<br />
P = Pn<br />
ρ<br />
51<br />
= =<br />
0. 91<br />
56 kW<br />
S = apparent power<br />
S = P 56<br />
= = 6 5 kVA<br />
cos ϕ 0. 86<br />
So that, on referring to diagram Figure L5 or using a pocket calculator, the value <strong>of</strong><br />
tan ϕ corresponding to a cos ϕ <strong>of</strong> 0.86 is found to be 0.59<br />
Q = P tan ϕ = 56 x 0.59 = 33 kvar (see Figure L15).<br />
Alternatively<br />
2 2 2 2<br />
Q = S -P = 65 - 56 = 33 kvar<br />
Average power factor values for the most commonly-used equipment and<br />
appliances (see Fig. L6)<br />
Equipment and appliances cos ϕ tan ϕ<br />
b Common loaded at 0% 0.17 5.80<br />
induction motor 25% 0.55 1.52<br />
50% 0.73 0.94<br />
75% 0.80 0.75<br />
100% 0.85 0.62<br />
b Incandescent lamps 1.0 0<br />
b Fluorescent lamps (uncompensated) 0.5 1.73<br />
b Fluorescent lamps (compensated) 0.93 0.39<br />
b Discharge lamps 0.4 to 0.6 2.29 to 1.33<br />
b Ovens using resistance elements 1.0 0<br />
b Induction heating ovens (compensated) 0.85 0.62<br />
b Dielectric type heating ovens 0.85 0.62<br />
b Resistance-type soldering machines 0.8 to 0.9 0.75 to 0.48<br />
b Fixed 1-phase arc-welding set 0.5 1.73<br />
b Arc-welding motor-generating set 0.7 to 0.9 1.02 to 0.48<br />
b Arc-welding transformer-rectifier set 0.7 to 0.8 1.02 to 0.75<br />
b Arc furnace 0.8 0.75<br />
Fig. L6 : Values <strong>of</strong> cos ϕ and tan ϕ for commonly-used equipment<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
An improvement <strong>of</strong> the power factor <strong>of</strong> an<br />
<strong>installation</strong> presents several technical and<br />
economic advantages, notably in the reduction<br />
<strong>of</strong> electricity bills<br />
Power factor improvement allows the use <strong>of</strong><br />
smaller transformers, switchgear and cables,<br />
etc. as well as reducing power losses and<br />
voltage drop in an <strong>installation</strong><br />
2 Why to improve the power<br />
factor?<br />
2.1 Reduction in the cost <strong>of</strong> electricity<br />
Good management in the consumption <strong>of</strong> reactive energy brings economic<br />
advantages.<br />
These notes are based on an actual tariff structure commonly applied in Europe,<br />
<strong>design</strong>ed to encourage consumers to minimize their consumption <strong>of</strong> reactive energy.<br />
The <strong>installation</strong> <strong>of</strong> power-factor correction capacitors on <strong>installation</strong>s permits the<br />
consumer to reduce his electricity bill by maintaining the level <strong>of</strong> reactive-power<br />
consumption below a value contractually agreed with the power supply authority.<br />
In this particular tariff, reactive energy is billed according to the tan ϕ criterion.<br />
As previously noted:<br />
tan ϕ =<br />
Q (kvarh)<br />
P (kWh)<br />
The power supply authority delivers reactive energy for free:<br />
b If the reactive energy represents less than 40% <strong>of</strong> the active energy (tan ϕ < 0.4)<br />
for a maximum period <strong>of</strong> 16 hours each day (from 06-00 h to 22-00 h) during the<br />
most-heavily loaded period (<strong>of</strong>ten in winter)<br />
b Without limitation during light-load periods in winter, and in spring and summer.<br />
During the periods <strong>of</strong> limitation, reactive energy consumption exceeding 40% <strong>of</strong><br />
the active energy (i.e. tan ϕ > 0.4) is billed monthly at the current rates. Thus, the<br />
quantity <strong>of</strong> reactive energy billed in these periods will be:<br />
kvarh (to be billed) = kWh (tan ϕ > 0.4) where:<br />
v kWh is the active energy consumed during the periods <strong>of</strong> limitation<br />
v kWh tan ϕ is the total reactive energy during a period <strong>of</strong> limitation<br />
v 0.4 kWh is the amount <strong>of</strong> reactive energy delivered free during a period <strong>of</strong><br />
limitation<br />
tan ϕ = 0.4 corresponds to a power factor <strong>of</strong> 0.93 so that, if steps are taken to ensure<br />
that during the limitation periods the power factor never falls below 0.93,<br />
the consumer will have nothing to pay for the reactive power consumed.<br />
Against the financial advantages <strong>of</strong> reduced billing, the consumer must balance<br />
the cost <strong>of</strong> purchasing, installing and maintaining the power factor improvement<br />
capacitors and controlling switchgear, automatic control equipment (where stepped<br />
levels <strong>of</strong> compensation are required) together with the additional kWh consumed by<br />
the dielectric losses <strong>of</strong> the capacitors, etc. It may be found that it is more economic<br />
to provide partial compensation only, and that paying for some <strong>of</strong> the reactive energy<br />
consumed is less expensive than providing 100% compensation.<br />
The question <strong>of</strong> power-factor correction is a matter <strong>of</strong> optimization, except in very<br />
simple cases.<br />
2.2 Technical/economic optimization<br />
A high power factor allows the optimization <strong>of</strong> the components <strong>of</strong> an <strong>installation</strong>.<br />
Overating <strong>of</strong> certain equipment can be avoided, but to achieve the best results, the<br />
correction should be effected as close to the individual inductive items as possible.<br />
Reduction <strong>of</strong> cable size<br />
Figure L7 shows the required increase in the size <strong>of</strong> cables as the power factor is<br />
reduced from unity to 0.4, for the same active power transmitted.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Multiplying factor 1 1.25 1.67 2.5<br />
for the cross-sectional<br />
area <strong>of</strong> the cable core(s)<br />
cos ϕ 1 0.8 0.6 0.4<br />
Fig. L7 : Multiplying factor for cable size as a function <strong>of</strong> cos ϕ<br />
L<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
L<br />
L - Power factor correction and<br />
harmonic filtering<br />
(1) Since other benefits are obtained from a high value <strong>of</strong><br />
power factor, as previously noted.<br />
2 Why to improve the power<br />
factor?<br />
Reduction <strong>of</strong> losses (P, kW) in cables<br />
Losses in cables are proportional to the current squared, and are measured by the<br />
kWh meter <strong>of</strong> the <strong>installation</strong>. Reduction <strong>of</strong> the total current in a conductor by 10% for<br />
example, will reduce the losses by almost 20%.<br />
Reduction <strong>of</strong> voltage drop<br />
Power factor correction capacitors reduce or even cancel completely the (inductive)<br />
reactive current in upstream conductors, thereby reducing or eliminating voltage<br />
drops.<br />
Note: Over compensation will produce a voltage rise at the capacitor level.<br />
Increase in available power<br />
By improving the power factor <strong>of</strong> a load supplied from a transformer, the current<br />
through the transformer will be reduced, thereby allowing more load to be added. In<br />
practice, it may be less expensive to improve the power factor (1) , than to replace the<br />
transformer by a larger unit.<br />
This matter is further elaborated in clause 6.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
Improving the power factor <strong>of</strong> an <strong>installation</strong><br />
requires a bank <strong>of</strong> capacitors which acts as a<br />
source <strong>of</strong> reactive energy. This arrangement is<br />
said to provide reactive energy compensation<br />
a) Reactive current components only flow pattern<br />
IL - IC IC IL IL<br />
C L<br />
Load<br />
b) When IC = IL, all reactive power is supplied from the<br />
capacitor bank<br />
IL - IC = 0 IC IL IL<br />
C L<br />
c) With load current added to case (b)<br />
Load<br />
IR IC IR + IL IL IR<br />
C L<br />
Load<br />
Fig. L8 : Showing the essential features <strong>of</strong> power-factor<br />
correction<br />
ϕ'<br />
ϕ<br />
Qc<br />
Fig. L9 : Diagram showing the principle <strong>of</strong> compensation:<br />
Qc = P (tan ϕ - tan ϕ’)<br />
S'<br />
S<br />
Q'<br />
P<br />
R<br />
R<br />
R<br />
Q<br />
3 How to improve the power factor?<br />
3.1 Theoretical principles<br />
An inductive load having a low power factor requires the generators and<br />
transmission/distribution systems to pass reactive current (lagging the system<br />
voltage by 90 degrees) with associated power losses and exaggerated voltage<br />
drops, as noted in sub-clause 1.1. If a bank <strong>of</strong> shunt capacitors is added to the<br />
load, its (capacitive) reactive current will take the same path through the power<br />
system as that <strong>of</strong> the load reactive current. Since, as pointed out in sub-clause<br />
1.1, this capacitive current Ic (which leads the system voltage by 90 degrees) is<br />
in direct phase opposition to the load reactive current (IL), the two components<br />
flowing through the same path will cancel each other, such that if the capacitor bank<br />
is sufficiently large and Ic = IL there will be no reactive current flow in the system<br />
upstream <strong>of</strong> the capacitors.<br />
This is indicated in Figure L8 (a) and (b) which show the flow <strong>of</strong> the reactive<br />
components <strong>of</strong> current only.<br />
In this figure:<br />
R represents the active-power elements <strong>of</strong> the load<br />
L represents the (inductive) reactive-power elements <strong>of</strong> the load<br />
C represents the (capacitive) reactive-power elements <strong>of</strong> the power-factor correction<br />
equipment (i.e. capacitors).<br />
It will be seen from diagram (b) <strong>of</strong> Figure L9, that the capacitor bank C appears<br />
to be supplying all the reactive current <strong>of</strong> the load. For this reason, capacitors are<br />
sometimes referred to as “generators <strong>of</strong> lagging vars”.<br />
In diagram (c) <strong>of</strong> Figure L9, the active-power current component has been added,<br />
and shows that the (fully-compensated) load appears to the power system as having<br />
a power factor <strong>of</strong> 1.<br />
In general, it is not economical to fully compensate an <strong>installation</strong>.<br />
Figure L9 uses the power diagram discussed in sub-clause 1.3 (see Fig. L3) to<br />
illustrate the principle <strong>of</strong> compensation by reducing a large reactive power Q to a<br />
smaller value Q’ by means <strong>of</strong> a bank <strong>of</strong> capacitors having a reactive power Qc.<br />
In doing so, the magnitude <strong>of</strong> the apparent power S is seen to reduce to S’.<br />
Example:<br />
A motor consumes 100 kW at a power factor <strong>of</strong> 0.75 (i.e. tan ϕ = 0.88). To improve<br />
the power factor to 0.93 (i.e. tan ϕ = 0.4), the reactive power <strong>of</strong> the capacitor bank<br />
must be : Qc = 100 (0.88 - 0.4) = 48 kvar<br />
The selected level <strong>of</strong> compensation and the calculation <strong>of</strong> rating for the capacitor<br />
bank depend on the particular <strong>installation</strong>. The factors requiring attention are<br />
explained in a general way in clause 5, and in clauses 6 and 7 for transformers and<br />
motors.<br />
Note: Before starting a compensation project, a number <strong>of</strong> precautions should be<br />
observed. In particular, oversizing <strong>of</strong> motors should be avoided, as well as the noload<br />
running <strong>of</strong> motors. In this latter condition, the reactive energy consumed by a<br />
motor results in a very low power factor (≈ 0.17); this is because the kW taken by the<br />
motor (when it is unloaded) are very small.<br />
3.2 By using what equipment?<br />
Compensation at LV<br />
At low voltage, compensation is provided by:<br />
b Fixed-value capacitor<br />
b Equipment providing automatic regulation, or banks which allow continuous<br />
adjustment according to requirements, as loading <strong>of</strong> the <strong>installation</strong> changes<br />
Note: When the installed reactive power <strong>of</strong> compensation exceeds 800 kvar, and the<br />
load is continuous and stable, it is <strong>of</strong>ten found to be economically advantageous to<br />
instal capacitor banks at the medium voltage level.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
L<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
L8<br />
L - Power factor correction and<br />
harmonic filtering<br />
Compensation can be carried out by a<br />
fixed value <strong>of</strong> capacitance in favourable<br />
circumstances<br />
Compensation is more-commonly effected by<br />
means <strong>of</strong> an automatically-controlled stepped<br />
bank <strong>of</strong> capacitors<br />
3 How to improve the power factor?<br />
Fixed capacitors (see Fig. L10)<br />
This arrangement employs one or more capacitor(s) to form a constant level <strong>of</strong><br />
compensation. Control may be:<br />
b Manual: by circuit-breaker or load-break switch<br />
b Semi-automatic: by contactor<br />
b Direct connection to an appliance and switched with it<br />
These capacitors are applied:<br />
b At the terminals <strong>of</strong> inductive devices (motors and transformers)<br />
b At busbars supplying numerous small motors and inductive appliance for which<br />
individual compensation would be too costly<br />
b In cases where the level <strong>of</strong> load is reasonably constant<br />
Fig. L10 : Example <strong>of</strong> fixed-value compensation capacitors<br />
Automatic capacitor banks (see Fig. L11)<br />
This kind <strong>of</strong> equipment provides automatic control <strong>of</strong> compensation, maintaining the<br />
power factor within close limits around a selected level. Such equipment is applied at<br />
points in an <strong>installation</strong> where the active-power and/or reactive-power variations are<br />
relatively large, for example:<br />
b At the busbars <strong>of</strong> a general power distribution board<br />
b At the terminals <strong>of</strong> a heavily-loaded feeder cable<br />
Fig. L11 : Example <strong>of</strong> automatic-compensation-regulating equipment<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
Automatically-regulated banks <strong>of</strong> capacitors<br />
allow an immediate adaptation <strong>of</strong> compensation<br />
to match the level <strong>of</strong> load<br />
3 How to improve the power factor?<br />
The principles <strong>of</strong>, and reasons, for using automatic<br />
compensation<br />
A bank <strong>of</strong> capacitors is divided into a number <strong>of</strong> sections, each <strong>of</strong> which is controlled<br />
by a contactor. Closure <strong>of</strong> a contactor switches its section into parallel operation with<br />
other sections already in service. The size <strong>of</strong> the bank can therefore be increased or<br />
decreased in steps, by the closure and opening <strong>of</strong> the controlling contactors.<br />
A control relay monitors the power factor <strong>of</strong> the controlled circuit(s) and is arranged<br />
to close and open appropriate contactors to maintain a reasonably constant<br />
system power factor (within the tolerance imposed by the size <strong>of</strong> each step <strong>of</strong><br />
compensation). The current transformer for the monitoring relay must evidently<br />
be placed on one phase <strong>of</strong> the incoming cable which supplies the circuit(s) being<br />
controlled, as shown in Figure L12.<br />
A Varset Fast capacitor bank is an automatic power factor correction equipment<br />
including static contactors (thyristors) instead <strong>of</strong> usual contactors. Static correction<br />
is particularly suitable for a certain number <strong>of</strong> <strong>installation</strong>s using equipment with fast<br />
cycle and/or sensitive to transient surges.<br />
The advantages <strong>of</strong> static contactors are :<br />
b Immediate response to all power factor fluctuation (response time 2 s or 40 ms<br />
according to regulator option)<br />
b Unlimited number <strong>of</strong> operations<br />
b Elimination <strong>of</strong> transient phenomena on the network on capacitor switching<br />
b Fully silent operation<br />
By closely matching compensation to that required by the load, the possibility <strong>of</strong><br />
producing overvoltages at times <strong>of</strong> low load will be avoided, thereby preventing<br />
an overvoltage condition, and possible damage to appliances and equipment.<br />
Overvoltages due to excessive reactive compensation depend partly on the value <strong>of</strong><br />
source impedance.<br />
CT In / 5 A cl 1<br />
Fig. L12 : The principle <strong>of</strong> automatic-compensation control<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Varmetric<br />
relay<br />
3.3 The choice between a fixed or automaticallyregulated<br />
bank <strong>of</strong> capacitors<br />
Commonly-applied <strong>rules</strong><br />
Where the kvar rating <strong>of</strong> the capacitors is less than, or equal to 15% <strong>of</strong> the supply<br />
transformer rating, a fixed value <strong>of</strong> compensation is appropriate. Above the 15%<br />
level, it is advisable to install an automatically-controlled bank <strong>of</strong> capacitors.<br />
The location <strong>of</strong> low-voltage capacitors in an <strong>installation</strong> constitutes the mode <strong>of</strong><br />
compensation, which may be global (one location for the entire <strong>installation</strong>), partial<br />
(section-by-section), local (at each individual device), or some combination <strong>of</strong> the<br />
latter two. In principle, the ideal compensation is applied at a point <strong>of</strong> consumption<br />
and at the level required at any instant.<br />
In practice, technical and economic factors govern the choice.<br />
L9<br />
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L10<br />
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L - Power factor correction and<br />
harmonic filtering<br />
Where a load is continuous and stable, global<br />
compensation can be applied<br />
Compensation by sector is recommended<br />
when the <strong>installation</strong> is extensive, and where the<br />
load/time patterns differ from one part <strong>of</strong><br />
the <strong>installation</strong> to another<br />
no. 1<br />
no. 2 no. 2<br />
M M M M<br />
Fig. L14 : Compensation by sector<br />
4 Where to install correction<br />
capacitors?<br />
4.1 Global compensation (see Fig. L13)<br />
Principle<br />
The capacitor bank is connected to the busbars <strong>of</strong> the main LV distribution board for<br />
the <strong>installation</strong>, and remains in service during the period <strong>of</strong> normal load.<br />
Advantages<br />
The global type <strong>of</strong> compensation:<br />
b Reduces the tariff penalties for excessive consumption <strong>of</strong> kvars<br />
b Reduces the apparent power kVA demand, on which standing charges are usually<br />
based<br />
b Relieves the supply transformer, which is then able to accept more load if<br />
necessary<br />
Comments<br />
b Reactive current still flows in all conductors <strong>of</strong> cables leaving (i.e. downstream <strong>of</strong>)<br />
the main LV distribution board<br />
b For the above reason, the sizing <strong>of</strong> these cables, and power losses in them, are<br />
not improved by the global mode <strong>of</strong> compensation.<br />
Fig. L13 : Global compensation<br />
4.2 Compensation by sector (see Fig. L14)<br />
Principle<br />
Capacitor banks are connected to busbars <strong>of</strong> each local distribution board, as shown<br />
in Figure L14.<br />
A significant part <strong>of</strong> the <strong>installation</strong> benefits from this arrangement, notably the feeder<br />
cables from the main distribution board to each <strong>of</strong> the local distribution boards at<br />
which the compensation measures are applied.<br />
Advantages<br />
The compensation by sector:<br />
b Reduces the tariff penalties for excessive consumption <strong>of</strong> kvars<br />
b Reduces the apparent power kVA demand, on which standing charges are usually<br />
based<br />
b Relieves the supply transformer, which is then able to accept more load if<br />
necessary<br />
b The size <strong>of</strong> the cables supplying the local distribution boards may be reduced, or<br />
will have additional capacity for possible load increases<br />
b Losses in the same cables will be reduced<br />
Comments<br />
b Reactive current still flows in all cables downstream <strong>of</strong> the local distribution boards<br />
b For the above reason, the sizing <strong>of</strong> these cables, and the power losses in them,<br />
are not improved by compensation by sector<br />
b Where large changes in loads occur, there is always a risk <strong>of</strong> overcompensation<br />
and consequent overvoltage problems<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
M M M M<br />
no.1
L - Power factor correction and<br />
harmonic filtering<br />
Individual compensation should be considered<br />
when the power <strong>of</strong> motor is significant with<br />
respect to power <strong>of</strong> the <strong>installation</strong><br />
4 Where to install correction<br />
capacitors?<br />
4.3 Individual compensation<br />
Principle<br />
Capacitors are connected directly to the terminals <strong>of</strong> inductive circuit (notably motors,<br />
see further in Clause 7). Individual compensation should be considered when the<br />
power <strong>of</strong> the motor is significant with respect to the declared power requirement<br />
(kVA) <strong>of</strong> the <strong>installation</strong>.<br />
The kvar rating <strong>of</strong> the capacitor bank is in the order <strong>of</strong> 25% <strong>of</strong> the kW rating <strong>of</strong> the<br />
motor. Complementary compensation at the origin <strong>of</strong> the <strong>installation</strong> (transformer)<br />
may also be beneficial.<br />
Advantages<br />
Individual compensation:<br />
b Reduces the tariff penalties for excessive consumption <strong>of</strong> kvars<br />
b Reduces the apparent power kVA demand<br />
b Reduces the size <strong>of</strong> all cables as well as the cable losses<br />
Comments<br />
b Significant reactive currents no longer exist in the <strong>installation</strong><br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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L12<br />
© Schneider Electric - all rights reserved<br />
L - Power factor correction and<br />
harmonic filtering<br />
5 How to decide the optimum level<br />
<strong>of</strong> compensation?<br />
5.1 <strong>General</strong> method<br />
Listing <strong>of</strong> reactive power demands at the <strong>design</strong> stage<br />
This listing can be made in the same way (and at the same time) as that for the<br />
power loading described in chapter A. The levels <strong>of</strong> active and reactive power<br />
loading, at each level <strong>of</strong> the <strong>installation</strong> (generally at points <strong>of</strong> distribution and subdistribution<br />
<strong>of</strong> circuits) can then be determined.<br />
Technical-economic optimization for an existing <strong>installation</strong><br />
The optimum rating <strong>of</strong> compensation capacitors for an existing <strong>installation</strong> can be<br />
determined from the following principal considerations:<br />
b Electricity bills prior to the <strong>installation</strong> <strong>of</strong> capacitors<br />
b Future electricity bills anticipated following the <strong>installation</strong> <strong>of</strong> capacitors<br />
b Costs <strong>of</strong>:<br />
v Purchase <strong>of</strong> capacitors and control equipment (contactors, relaying, cabinets, etc.)<br />
v Installation and maintenance costs<br />
v Cost <strong>of</strong> dielectric heating losses in the capacitors, versus reduced losses in cables,<br />
transformer, etc., following the <strong>installation</strong> <strong>of</strong> capacitors<br />
Several simplified methods applied to typical tariffs (common in Europe) are shown<br />
in sub-clauses 5.3 and 5.4.<br />
5.2 Simplified method<br />
<strong>General</strong> principle<br />
An approximate calculation is generally adequate for most practical cases, and may<br />
be based on the assumption <strong>of</strong> a power factor <strong>of</strong> 0.8 (lagging) before compensation.<br />
In order to improve the power factor to a value sufficient to avoid tariff penalties (this<br />
depends on local tariff structures, but is assumed here to be 0.93) and to reduce<br />
losses, volt-drops, etc. in the <strong>installation</strong>, reference can be made to Figure L15 next<br />
page.<br />
From the figure, it can be seen that, to raise the power factor <strong>of</strong> the <strong>installation</strong> from<br />
0.8 to 0.93 will require 0.355 kvar per kW <strong>of</strong> load. The rating <strong>of</strong> a bank <strong>of</strong> capacitors<br />
at the busbars <strong>of</strong> the main distribution board <strong>of</strong> the <strong>installation</strong> would be<br />
Q (kvar) = 0.355 x P (kW).<br />
This simple approach allows a rapid determination <strong>of</strong> the compensation capacitors<br />
required, albeit in the global, partial or independent mode.<br />
Example<br />
It is required to improve the power factor <strong>of</strong> a 666 kVA <strong>installation</strong> from 0.75 to 0.928.<br />
The active power demand is 666 x 0.75 = 500 kW.<br />
In Figure L15, the intersection <strong>of</strong> the row cos ϕ = 0.75 (before correction) with<br />
the column cos ϕ = 0.93 (after correction) indicates a value <strong>of</strong> 0.487 kvar <strong>of</strong><br />
compensation per kW <strong>of</strong> load.<br />
For a load <strong>of</strong> 500 kW, therefore, 500 x 0.487 = 244 kvar <strong>of</strong> capacitive compensation<br />
is required.<br />
Note: this method is valid for any voltage level, i.e. is independent <strong>of</strong> voltage.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
5 How to decide the optimum level<br />
<strong>of</strong> compensation?<br />
Before kvar rating <strong>of</strong> capacitor bank to install per kW <strong>of</strong> load, to improve cos ϕ (the power factor) or tan ϕ,<br />
compensation to a given value<br />
tan ϕ 0.75 0.59 0.48 0.46 0.43 0.40 0.36 0.33 0.29 0.25 0.20 0.14 0.0<br />
tan ϕ cos ϕ cos ϕ 0.80 0.86 0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1<br />
2.29 0.40 1.557 1.691 1.805 1.832 1.861 1.895 1.924 1.959 1.998 2.037 2.085 2.146 2.288<br />
2.22 0.41 1.474 1.625 1.742 1.769 1.798 1.831 1.840 1.896 1.935 1.973 2.021 2.082 2.225<br />
2.16 0.42 1.413 1.561 1.681 1.709 1.738 1.771 1.800 1.836 1.874 1.913 1.961 2.022 2.164<br />
2.10 0.43 1.356 1.499 1.624 1.651 1.680 1.713 1.742 1.778 1.816 1.855 1.903 1.964 2.107<br />
2.04 0.44 1.290 1.441 1.558 1.585 1.614 1.647 1.677 1.712 1.751 1.790 1.837 1.899 2.041<br />
1.98 0.45 1.230 1.384 1.501 1.532 1.561 1.592 1.628 1.659 1.695 1.737 1.784 1.846 1.988<br />
1.93 0.46 1.179 1.330 1.446 1.473 1.502 1.533 1.567 1.600 1.636 1.677 1.725 1.786 1.929<br />
1.88 0.47 1.130 1.278 1.397 1.425 1.454 1.485 1.519 1.532 1.588 1.629 1.677 1.758 1.881<br />
1.83 0.48 1.076 1.228 1.343 1.370 1.400 1.430 1.464 1.497 1.534 1.575 1.623 1.684 1.826<br />
1.78 0.49 1.030 1.179 1.297 1.326 1.355 1.386 1.420 1.453 1.489 1.530 1.578 1.639 1.782<br />
1.73 0.50 0.982 1.232 1.248 1.276 1.303 1.337 1.369 1.403 1.441 1.481 1.529 1.590 1.732<br />
1.69 0.51 0.936 1.087 1.202 1.230 1.257 1.291 1.323 1.357 1.395 1.435 1.483 1.544 1.686<br />
1.64 0.52 0.894 1.043 1.160 1.188 1.215 1.249 1.281 1.315 1.353 1.393 1.441 1.502 1.644<br />
1.60 0.53 0.850 1.000 1.116 1.144 1.171 1.205 1.237 1.271 1.309 1.349 1.397 1.458 1.600<br />
1.56 0.54 0.809 0.959 1.075 1.103 1.130 1.164 1.196 1.230 1.268 1.308 1.356 1.417 1.559<br />
1.52 0.55 0.769 0.918 1.035 1.063 1.090 1.124 1.156 1.190 1.228 1.268 1.316 1.377 1.519<br />
1.48 0.56 0.730 0.879 0.996 1.024 1.051 1.085 1.117 1.151 1.189 1.229 1.277 1.338 1.480<br />
1.44 0.57 0.692 0.841 0.958 0.986 1.013 1.047 1.079 1.113 1.151 1.191 1.239 1.300 1.442<br />
1.40 0.58 0.665 0.805 0.921 0.949 0.976 1.010 1.042 1.076 1.114 1.154 1.202 1.263 1.405<br />
1.37 0.59 0.618 0.768 0.884 0.912 0.939 0.973 1.005 1.039 1.077 1.117 1.165 1.226 1.368<br />
1.33 0.60 0.584 0.733 0.849 0.878 0.905 0.939 0.971 1.005 1.043 1.083 1.131 1.192 1.334<br />
1.30 0.61 0.549 0.699 0.815 0.843 0.870 0.904 0.936 0.970 1.008 1.048 1.096 1.157 1.299<br />
1.27 0.62 0.515 0.665 0.781 0.809 0.836 0.870 0.902 0.936 0.974 1.014 1.062 1.123 1.265<br />
1.23 0.63 0.483 0.633 0.749 0.777 0.804 0.838 0.870 0.904 0.942 0.982 1.030 1.091 1.233<br />
1.20 0.64 0.450 0.601 0.716 0.744 0.771 0.805 0.837 0.871 0.909 0.949 0.997 1.058 1.200<br />
1.17 0.65 0.419 0.569 0.685 0.713 0.740 0.774 0.806 0.840 0.878 0.918 0.966 1.007 1.169<br />
1.14 0.66 0.388 0.538 0.654 0.682 0.709 0.743 0.775 0.809 0.847 0.887 0.935 0.996 1.138<br />
1.11 0.67 0.358 0.508 0.624 0.652 0.679 0.713 0.745 0.779 0.817 0.857 0.905 0.966 1.108<br />
1.08 0.68 0.329 0.478 0.595 0.623 0.650 0.684 0.716 0.750 0.788 0.828 0.876 0.937 1.079<br />
1.05 0.69 0.299 0.449 0.565 0.593 0.620 0.654 0.686 0.720 0.758 0.798 0.840 0.907 1.049<br />
1.02 0.70 0.270 0.420 0.536 0.564 0.591 0.625 0.657 0.691 0.729 0.769 0.811 0.878 1.020<br />
0.99 0.71 0.242 0.392 0.508 0.536 0.563 0.597 0.629 0.663 0.701 0.741 0.783 0.850 0.992<br />
0.96 0.72 0.213 0.364 0.479 0.507 0.534 0.568 0.600 0.634 0.672 0.712 0.754 0.821 0.963<br />
0.94 0.73 0.186 0.336 0.452 0.480 0.507 0.541 0.573 0.607 0.645 0.685 0.727 0.794 0.936<br />
0.91 0.74 0.159 0.309 0.425 0.453 0.480 0.514 0.546 0.580 0.618 0.658 0.700 0.767 0.909<br />
0.88 0.75 0.132 0.82 0.398 0.426 0.453 0.487 0.519 0.553 0.591 0.631 0.673 0.740 0.882<br />
0.86 0.76 0.105 0.255 0.371 0.399 0.426 0.460 0.492 0.526 0.564 0.604 0.652 0.713 0.855<br />
0.83 0.77 0.079 0.229 0.345 0.373 0.400 0.434 0.466 0.500 0.538 0.578 0.620 0.687 0.829<br />
0.80 0.78 0.053 0.202 0.319 0.347 0.374 0.408 0.440 0.474 0.512 0.552 0.594 0.661 0.803<br />
0.78 0.79 0.026 0.176 0.292 0.320 0.347 0.381 0.413 0.447 0.485 0.525 0.567 0.634 0.776<br />
0.75 0.80 0.150 0.266 0.294 0.321 0.355 0.387 0.421 0.459 0.499 0.541 0.608 0.750<br />
0.72 0.81 0.124 0.240 0.268 0.295 0.329 0.361 0.395 0.433 0.473 0.515 0.582 0.724<br />
0.70 0.82 0.098 0.214 0.242 0.269 0.303 0.335 0.369 0.407 0.447 0.489 0.556 0.698<br />
0.67 0.83 0.072 0.188 0.216 0.243 0.277 0.309 0.343 0.381 0.421 0.463 0.530 0.672<br />
0.65 0.84 0.046 0.162 0.190 0.217 0.251 0.283 0.317 0.355 0.395 0.437 0.504 0.645<br />
0.62 0.85 0.020 0.136 0.164 0.191 0.225 0.257 0.291 0.329 0.369 0.417 0.478 0.620<br />
0.59 0.86 0.109 0.140 0.167 0.198 0.230 0.264 0.301 0.343 0.390 0.450 0.593<br />
0.57 0.87 0.083 0.114 0.141 0.172 0.204 0.238 0.275 0.317 0.364 0.424 0.567<br />
0.54 0.88 0.054 0.085 0.112 0.143 0.175 0.209 0.246 0.288 0.335 0.395 0.538<br />
0.51 0.89 0.028 0.059 0.086 0.117 0.149 0.183 0.230 0.262 0.309 0.369 0.512<br />
0.48 0.90 0.031 0.058 0.089 0.121 0.155 0.192 0.234 0.281 0.341 0.484<br />
Value selected as an example on section 5.2<br />
Value selected as an example on section 5.4<br />
Fig. L15 : kvar to be installed per kW <strong>of</strong> load, to improve the power factor <strong>of</strong> an <strong>installation</strong><br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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L14<br />
© Schneider Electric - all rights reserved<br />
L - Power factor correction and<br />
harmonic filtering<br />
In the case <strong>of</strong> certain (common) types <strong>of</strong><br />
tariff, an examination <strong>of</strong> several bills covering<br />
the most heavily-loaded period <strong>of</strong> the year<br />
allows determination <strong>of</strong> the kvar level <strong>of</strong><br />
compensation required to avoid kvarh (reactiveenergy)<br />
charges. The pay-back period <strong>of</strong> a<br />
bank <strong>of</strong> power-factor-correction capacitors<br />
and associated equipment is generally about<br />
18 months<br />
For 2-part tariffs based partly on a declared value<br />
<strong>of</strong> kVA, Figure L17 allows determination <strong>of</strong> the<br />
kvar <strong>of</strong> compensation required to reduce the<br />
value <strong>of</strong> kVA declared, and to avoid exceeding it<br />
ϕ'<br />
ϕ<br />
Cos ϕ = 0.7<br />
Cos ϕ'= 0.95<br />
S = 122 kVA<br />
S' = 90 kVA<br />
Q = 87.1 kvar<br />
Qc = 56 kvar<br />
Q' = 28.1 kvar<br />
P = 85.4 kW<br />
Fig. L16 : Reduction <strong>of</strong> declared maximum kVA by powerfactor<br />
improvement<br />
(1) In the billing period, during the hours for which<br />
reactive energy is charged for the case considered above:<br />
15,996 kvarh<br />
Qc = =<br />
73 kvar<br />
220 h<br />
S'<br />
S<br />
Q'<br />
Qc<br />
Q<br />
5 How to decide the optimum level<br />
<strong>of</strong> compensation?<br />
5.3 Method based on the avoidance <strong>of</strong> tariff<br />
penalties<br />
The following method allows calculation <strong>of</strong> the rating <strong>of</strong> a proposed capacitor bank,<br />
based on billing details, where the tariff structure corresponds with (or is similar to)<br />
the one described in sub-clause 2.1 <strong>of</strong> this chapter.<br />
The method determines the minimum compensation required to avoid these charges<br />
which are based on kvarh consumption.<br />
The procedure is as follows:<br />
b Refer to the bills covering consumption for the 5 months <strong>of</strong> winter (in France these<br />
are November to March inclusive).<br />
Note: in tropical climates the summer months may constitute the period <strong>of</strong> heaviest<br />
loading and highest peaks (owing to extensive air conditioning loads) so that a<br />
consequent variation <strong>of</strong> high-tariff periods is necessary in this case. The remainder <strong>of</strong><br />
this example will assume Winter conditions in France.<br />
b Identify the line on the bills referring to “reactive-energy consumed” and “kvarh<br />
to be charged”. Choose the bill which shows the highest charge for kvarh (after<br />
checking that this was not due to some exceptional situation).<br />
For example: 15,966 kvarh in January.<br />
b Evaluate the total period <strong>of</strong> loaded operation <strong>of</strong> the <strong>installation</strong> for that month, for<br />
instance: 220 hours (22 days x 10 hours). The hours which must be counted are<br />
those occurring during the heaviest load and the highest peak loads occurring on<br />
the power system. These are given in the tariff documents, and are (commonly)<br />
during a 16-hour period each day, either from 06.00 h to 22.00 h or from 07.00 h to<br />
23.00 h according to the region. Outside these periods, no charge is made for kvarh<br />
consumption.<br />
b The necessary value <strong>of</strong> compensation in kvar = kvarh billed/number <strong>of</strong> hours <strong>of</strong><br />
operation (1) = Qc<br />
The rating <strong>of</strong> the installed capacitor bank is generally chosen to be slightly larger<br />
than that calculated.<br />
Certain manufacturers can provide “slide <strong>rules</strong>” especially <strong>design</strong>ed to facilitate<br />
these kinds <strong>of</strong> calculation, according to particular tariffs. These devices and<br />
accompanying documentation advice on suitable equipment and control schemes,<br />
as well as drawing attention to constraints imposed by harmonic voltages on the<br />
power system. Such voltages require either over dimensioned capacitors (in terms <strong>of</strong><br />
heat-dissipation, voltage and current ratings) and/or harmonic-suppression inductors<br />
or filters.<br />
5.4 Method based on reduction <strong>of</strong> declared<br />
maximum apparent power (kVA)<br />
For consumers whose tariffs are based on a fixed charge per kVA declared, plus a<br />
charge per kWh consumed, it is evident that a reduction in declared kVA would be<br />
beneficial. The diagram <strong>of</strong> Figure L16 shows that as the power factor improves, the<br />
kVA value diminishes for a given value <strong>of</strong> kW (P). The improvement <strong>of</strong> the power<br />
factor is aimed at (apart from other advantages previously mentioned) reducing the<br />
declared level and never exceeding it, thereby avoiding the payment <strong>of</strong> an excessive<br />
price per kVA during the periods <strong>of</strong> excess, and/or tripping <strong>of</strong> the the main circuitbreaker.<br />
Figure L15 (previous page) indicates the value <strong>of</strong> kvar <strong>of</strong> compensation per<br />
kW <strong>of</strong> load, required to improve from one value <strong>of</strong> power factor to another.<br />
Example:<br />
A supermarket has a declared load <strong>of</strong> 122 kVA at a power factor <strong>of</strong> 0.7 lagging, i.e.an<br />
active-power load <strong>of</strong> 85.4 kW. The particular contract for this consumer was based on<br />
stepped values <strong>of</strong> declared kVA (in steps <strong>of</strong> 6 kVA up to 108 kVA, and 12 kVA steps<br />
above that value, this is a common feature in many types <strong>of</strong> two-part tariff). In the<br />
case being considered, the consumer was billed on the basis <strong>of</strong><br />
132 kVA. Referring to Figure L15, it can be seen that a 60 kvar bank <strong>of</strong> capacitors<br />
will improve the power factor <strong>of</strong> the load from 0.7 to 0.95 (0.691 x 85.4 = 59 kvar<br />
in the figure). The declared value <strong>of</strong> kVA will then be 85.4<br />
= 9 0 kVA , i.e. an<br />
improvement <strong>of</strong> 30%.<br />
0.95<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
The <strong>installation</strong> <strong>of</strong> a capacitor bank can avoid<br />
the need to change a transformer in the event <strong>of</strong><br />
a load increase<br />
6 Compensation at the terminals<br />
<strong>of</strong> a transformer<br />
6.1 Compensation to increase the available active<br />
power output<br />
Steps similar to those taken to reduce the declared maximum kVA, i.e. improvement<br />
<strong>of</strong> the load power factor, as discussed in subclause 5.4, will maximise the available<br />
transformer capacity, i.e. to supply more active power.<br />
Cases can arise where the replacement <strong>of</strong> a transformer by a larger unit, to overcome<br />
a load growth, may be avoided by this means. Figure L17 shows directly the power<br />
(kW) capability <strong>of</strong> fully-loaded transformers at different load power factors, from<br />
which the increase <strong>of</strong> active-power output can be obtained as the value <strong>of</strong> power<br />
factor increases.<br />
Fig. L17 : Active-power capability <strong>of</strong> fully-loaded transformers, when supplying loads at different values <strong>of</strong> power factor<br />
Q<br />
P1<br />
tan ϕ cos ϕ Nominal rating <strong>of</strong> transformers (in kVA)<br />
100 160 250 315 400 500 630 800 1000 1250 1600 2000<br />
0.00 1 100 160 250 315 400 500 630 800 1000 1250 1600 2000<br />
0.20 0.98 98 157 245 309 392 490 617 784 980 1225 1568 1960<br />
0.29 0.96 96 154 240 302 384 480 605 768 960 1200 1536 1920<br />
0.36 0.94 94 150 235 296 376 470 592 752 940 1175 1504 1880<br />
0.43 0.92 92 147 230 290 368 460 580 736 920 1150 1472 1840<br />
0.48 0.90 90 144 225 284 360 450 567 720 900 1125 1440 1800<br />
0.54 0.88 88 141 220 277 352 440 554 704 880 1100 1408 1760<br />
0.59 0.86 86 138 215 271 344 430 541 688 860 1075 1376 1720<br />
0.65 0.84 84 134 210 265 336 420 529 672 840 1050 1344 1680<br />
0.70 0.82 82 131 205 258 328 410 517 656 820 1025 1312 1640<br />
0.75 0.80 80 128 200 252 320 400 504 640 800 1000 1280 1600<br />
0.80 0.78 78 125 195 246 312 390 491 624 780 975 1248 1560<br />
0.86 0.76 76 122 190 239 304 380 479 608 760 950 1216 1520<br />
0.91 0.74 74 118 185 233 296 370 466 592 740 925 1184 1480<br />
0.96 0.72 72 115 180 227 288 360 454 576 720 900 1152 1440<br />
1.02 0.70 70 112 175 220 280 350 441 560 700 875 1120 1400<br />
S1<br />
S<br />
S2 Q2<br />
Q1<br />
P2<br />
Q<br />
Q m<br />
Fig. L18 : Compensation Q allows the <strong>installation</strong>-load<br />
extension S2 to be added, without the need to replace the<br />
existing transformer, the output <strong>of</strong> which is limited to S<br />
P<br />
Example: (see Fig. L18 )<br />
An <strong>installation</strong> is supplied from a 630 kVA transformer loaded at 450 kW (P1) with a<br />
450<br />
mean power factor <strong>of</strong> 0.8 lagging. The apparent power S1=<br />
= 5 62 kVA<br />
0.8<br />
The corresponding reactive power<br />
2 2<br />
Q1= S1 − P1 = 337 kvar<br />
The anticipated load increase P2 = 100 kW at a power factor <strong>of</strong> 0.7 lagging.<br />
The apparent power 100<br />
S2<br />
= = 143 kVA<br />
The corresponding reactive 0.7 power<br />
2 2<br />
Q2 = S2 − P2 = 1 02 kvar<br />
What is the minimum value <strong>of</strong> capacitive kvar to be installed, in order to avoid a<br />
change <strong>of</strong> transformer?<br />
Total power now to be supplied:<br />
P = P1 + P2 = 550 kW<br />
The maximum reactive power capability <strong>of</strong> the 630 kVA transformer when delivering<br />
550 kW is:<br />
2 2<br />
2 2<br />
Qm = S − P Qm = 630 − 550 = 307 kvar<br />
Total reactive power required by the <strong>installation</strong> before compensation:<br />
Q1 + Q2 = 337 + 102 = 439 kvar<br />
So that the minimum size <strong>of</strong> capacitor bank to install:<br />
Qkvar = 439 - 307 = 132 kvar<br />
It should be noted that this calculation has not taken account <strong>of</strong> load peaks and their<br />
duration.<br />
The best possible improvement, i.e. correction which attains a power factor <strong>of</strong><br />
1 would permit a power reserve for the transformer <strong>of</strong> 630 - 550 = 80 kW.<br />
The capacitor bank would then have to be rated at 439 kvar.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
L15<br />
© Schneider Electric - all rights reserved
L16<br />
© Schneider Electric - all rights reserved<br />
L - Power factor correction and<br />
harmonic filtering<br />
Where metering is carried out at the MV side<br />
<strong>of</strong> a transformer, the reactive-energy losses in<br />
the transformer may need to be compensated<br />
(depending on the tariff)<br />
Perfect transformer<br />
Primary<br />
winding<br />
Secondary<br />
winding<br />
Fig. L19 : Transformer reactances per phase<br />
Leakage reactance<br />
Magnetizing<br />
reactance<br />
The reactive power absorbed by a transformer<br />
cannot be neglected, and can amount to (about)<br />
5% <strong>of</strong> the transformer rating when supplying<br />
its full load. Compensation can be provided by<br />
a bank <strong>of</strong> capacitors. In transformers, reactive<br />
power is absorbed by both shunt (magnetizing)<br />
and series (leakage flux) reactances. Complete<br />
compensation can be provided by a bank <strong>of</strong><br />
shunt-connected LV capacitors<br />
I sin<br />
'<br />
I sin<br />
'<br />
I<br />
E<br />
Source<br />
XL<br />
V<br />
Load<br />
Fig. L20 : Reactive power absorption by series inductance<br />
I<br />
V<br />
E<br />
IXL<br />
6 Compensation at the terminals<br />
<strong>of</strong> a transformer<br />
6.2 Compensation <strong>of</strong> reactive energy absorbed by<br />
the transformer<br />
The nature <strong>of</strong> transformer inductive reactances<br />
All previous references have been to shunt connected devices such as those used<br />
in normal loads, and power factor-correcting capacitor banks etc. The reason for<br />
this is that shunt connected equipment requires (by far) the largest quantities <strong>of</strong><br />
reactive energy in power systems; however, series-connected reactances, such as<br />
the inductive reactances <strong>of</strong> power lines and the leakage reactance <strong>of</strong> transformer<br />
windings, etc., also absorb reactive energy.<br />
Where metering is carried out at the MV side <strong>of</strong> a transformer, the reactive-energy<br />
losses in the transformer may (depending on the tariff) need to be compensated. As<br />
far as reactive-energy losses only are concerned, a transformer may be represented<br />
by the elementary diagram <strong>of</strong> Figure L19. All reactance values are referred to<br />
the secondary side <strong>of</strong> the transformer, where the shunt branch represents the<br />
magnetizing-current path. The magnetizing current remains practically constant (at<br />
about 1.8% <strong>of</strong> full-load current) from no load to full load, in normal circumstances,<br />
i.e. with a constant primary voltage, so that a shunt capacitor <strong>of</strong> fixed value can be<br />
installed at the MV or LV side, to compensate for the reactive energy absorbed.<br />
Reactive-power absorption in series-connected<br />
(leakage flux) reactance XL<br />
A simple illustration <strong>of</strong> this phenomenon is given by the vector diagram <strong>of</strong><br />
Figure L20.<br />
The reactive-current component through the load = I sin ϕ so that QL = VI sin ϕ.<br />
The reactive-current component from the source = I sin ϕ’ so that QE = EI sin ϕ’.<br />
It can be seen that E > V and sin ϕ’ > sin ϕ.<br />
The difference between EI sin ϕ’ and VI sin ϕ gives the kvar per phase absorbed<br />
by XL.<br />
It can be shown that this kvar value is equal to I2XL (which is analogous to the I2R active power (kW) losses due to the series resistance <strong>of</strong> power lines, etc.).<br />
From the I2XL formula it is very simple to deduce the kvar absorbed at any load value<br />
for a given transformer, as follows:<br />
If per-unit values are used (instead <strong>of</strong> percentage values) direct multiplication <strong>of</strong> I<br />
and XL can be carried out.<br />
Example:<br />
A 630 kVA transformer with a short-circuit reactance voltage <strong>of</strong> 4% is fully loaded.<br />
What is its reactive-power (kvar) loss?<br />
4% = 0.04 pu Ipu = 1<br />
loss = I 2 XL = 1 2 x 0.04 = 0.04 pu kvar<br />
where 1 pu = 630 kVA<br />
The 3-phase kvar losses are 630 x 0.04 = 25.2 kvar (or, quite simply, 4% <strong>of</strong> 630 kVA).<br />
At half load i.e. I = 0.5 pu the losses will be<br />
0.5 2 x 0.04 = 0.01 pu = 630 x 0.01 = 6.3 kvar and so on...<br />
This example, and the vector diagram <strong>of</strong> Figure L20 show that:<br />
b The power factor at the primary side <strong>of</strong> a loaded transformer is different (normally<br />
lower) than that at the secondary side (due to the absorption <strong>of</strong> vars)<br />
b Full-load kvar losses due to leakage reactance are equal to the transformer<br />
percentage reactance (4% reactance means a kvar loss equal to 4% <strong>of</strong> the kVA rating<br />
<strong>of</strong> the transformer)<br />
b kvar losses due to leakage reactance vary according to the current<br />
(or kVA loading) squared<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
6 Compensation at the terminals<br />
<strong>of</strong> a transformer<br />
To determine the total kvar losses <strong>of</strong> a transformer the constant magnetizing-current<br />
circuit losses (approx. 1.8% <strong>of</strong> the transformer kVA rating) must be added to the<br />
foregoing “series” losses. Figure L21 shows the no-load and full-load kvar losses for<br />
typical distribution transformers. In principle, series inductances can be compensated<br />
by fixed series capacitors (as is commonly the case for long MV transmission lines).<br />
This arrangement is operationally difficult, however, so that, at the voltage levels<br />
covered by this guide, shunt compensation is always applied.<br />
In the case <strong>of</strong> MV metering, it is sufficient to raise the power factor to a point where<br />
the transformer plus load reactive-power consumption is below the level at which a<br />
billing charge is made. This level depends on the tariff, but <strong>of</strong>ten corresponds to a<br />
tan ϕ value <strong>of</strong> 0.31 (cos ϕ <strong>of</strong> 0.955).<br />
Rated power (kVA) Reactive power (kvar) to be compensated<br />
No load Full load<br />
100 2.5 6.1<br />
160 3.7 9.6<br />
250 5.3 14.7<br />
315 6.3 18.4<br />
400 7.6 22.9<br />
500 9.5 28.7<br />
630 11.3 35.7<br />
800 20 54.5<br />
1000 23.9 72.4<br />
1250 27.4 94.5<br />
1600 31.9 126<br />
2000 37.8 176<br />
Fig. L21 : Reactive power consumption <strong>of</strong> distribution transformers with 20 kV primary windings<br />
As a matter <strong>of</strong> interest, the kvar losses in a transformer can be completely<br />
compensated by adjusting the capacitor bank to give the load a (slightly) leading<br />
power factor. In such a case, all <strong>of</strong> the kvar <strong>of</strong> the transformer is being supplied from<br />
the capacitor bank, while the input to the MV side <strong>of</strong> the transformer is at unity power<br />
factor, as shown in Figure L22.<br />
Load<br />
current<br />
I<br />
ϕ<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
E (Input voltage)<br />
V (Load voltage)<br />
I0 Compensation current<br />
Fig. L22 : Overcompensation <strong>of</strong> load to completely compensate transformer reactive-power losses<br />
In practical terms, therefore, compensation for transformer-absorbed kvar is included<br />
in the capacitors primarily intended for powerfactor correction <strong>of</strong> the load, either<br />
globally, partially, or in the individual mode. Unlike most other kvar-absorbing items,<br />
the transformer absorption (i.e. the part due to the leakage reactance) changes<br />
significantly with variations <strong>of</strong> load level, so that, if individual compensation is applied<br />
to the transformer, then an average level <strong>of</strong> loading will have to be assumed.<br />
Fortunately, this kvar consumption generally forms only a relatively small part <strong>of</strong> the<br />
total reactive power <strong>of</strong> an <strong>installation</strong>, and so mismatching <strong>of</strong> compensation at times<br />
<strong>of</strong> load change is not likely to be a problem.<br />
Figure L21 indicates typical kvar loss values for the magnetizing circuit (“no-load<br />
kvar” columns), as well as for the total losses at full load, for a standard range <strong>of</strong><br />
distribution transformers supplied at 20 kV (which include the losses due to the<br />
leakage reactance).<br />
IXL<br />
L17<br />
© Schneider Electric - all rights reserved
L18<br />
© Schneider Electric - all rights reserved<br />
L - Power factor correction and<br />
harmonic filtering<br />
Individual motor compensation is recommended<br />
where the motor power (kVA) is large with<br />
respect to the declared power <strong>of</strong> the <strong>installation</strong><br />
Before<br />
compensation<br />
Transformer<br />
Active<br />
power<br />
Motor<br />
M M<br />
After<br />
compensation<br />
Power<br />
made<br />
available<br />
C<br />
Reactive<br />
power<br />
supplied<br />
by capacitor<br />
7 Power factor correction <strong>of</strong><br />
induction motors<br />
7.1 Connection <strong>of</strong> a capacitor bank and protection<br />
settings<br />
<strong>General</strong> precautions<br />
Because <strong>of</strong> the small kW consumption, the power factor <strong>of</strong> a motor is very low at noload<br />
or on light load. The reactive current <strong>of</strong> the motor remains practically constant at<br />
all loads, so that a number <strong>of</strong> unloaded motors constitute a consumption <strong>of</strong> reactive<br />
power which is generally detrimental to an <strong>installation</strong>, for reasons explained in<br />
preceding sections.<br />
Two good general <strong>rules</strong> therefore are that unloaded motors should be switched <strong>of</strong>f,<br />
and motors should not be oversized (since they will then be lightly loaded).<br />
Connection<br />
The bank <strong>of</strong> capacitors should be connected directly to the terminals <strong>of</strong> the motor.<br />
Special motors<br />
It is recommended that special motors (stepping, plugging, inching, reversing motors,<br />
etc.) should not be compensated.<br />
Effect on protection settings<br />
After applying compensation to a motor, the current to the motor-capacitor<br />
combination will be lower than before, assuming the same motor-driven load<br />
conditions. This is because a significant part <strong>of</strong> the reactive component <strong>of</strong> the motor<br />
current is being supplied from the capacitor, as shown in Figure L23.<br />
Where the overcurrent protection devices <strong>of</strong> the motor are located upstream <strong>of</strong> the<br />
motor capacitor connection (and this will always be the case for terminal-connected<br />
capacitors), the overcurrent relay settings must be reduced in the ratio:<br />
cos ϕ before compensation / cos ϕ after compensation<br />
For motors compensated in accordance with the kvar values indicated in Figure L24<br />
(maximum values recommended for avoidance <strong>of</strong> self-excitation <strong>of</strong> standard<br />
induction motors, as discussed in sub-clause 7.2), the above-mentioned ratio<br />
will have a value similar to that indicated for the corresponding motor speed in<br />
Figure L25.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
3-phase motors 230/400 V<br />
Nominal power kvar to be installed<br />
Speed <strong>of</strong> rotation (rpm)<br />
kW hp 3000 1500 1000 750<br />
22 30 6 8 9 10<br />
30 40 7.5 10 11 12.5<br />
37 50 9 11 12.5 16<br />
45 60 11 13 14 17<br />
55 75 13 17 18 21<br />
75 100 17 22 25 28<br />
90 125 20 25 27 30<br />
110 150 24 29 33 37<br />
132 180 31 36 38 43<br />
160 218 35 41 44 52<br />
200 274 43 47 53 61<br />
250 340 52 57 63 71<br />
280 380 57 63 70 79<br />
355 482 67 76 86 98<br />
400 544 78 82 97 106<br />
450 610 87 93 107 117<br />
Figure L24 : Maximum kvar <strong>of</strong> power factor correction applicable to motor terminals without risk<br />
<strong>of</strong> self excitation<br />
Speed in rpm Reduction factor<br />
750 0.88<br />
1000 0.90<br />
1500 0.91<br />
3000 0.93<br />
Fig. L23 : Before compensation, the transformer supplies all<br />
the reactive power; after compensation, the capacitor supplies<br />
a large part <strong>of</strong> the reactive power Fig. L25 : Reduction factor for overcurrent protection after compensation
L - Power factor correction and<br />
harmonic filtering<br />
When a capacitor bank is connected to the<br />
terminals <strong>of</strong> an induction motor, it is important<br />
to check that the size <strong>of</strong> the bank is less than<br />
that at which self-excitation can occur<br />
Fig. L26 : Connection <strong>of</strong> the capacitor bank to the motor<br />
7 Power factor correction <strong>of</strong><br />
induction motors<br />
7.2 How self-excitation <strong>of</strong> an induction motor can be<br />
avoided<br />
When a motor is driving a high-inertia load, the motor will continue to rotate (unless<br />
deliberately braked) after the motor supply has been switched <strong>of</strong>f.<br />
The “magnetic inertia” <strong>of</strong> the rotor circuit means that an emf will be generated in the<br />
stator windings for a short period after switching <strong>of</strong>f, and would normally reduce to<br />
zero after 1 or 2 cycles, in the case <strong>of</strong> an uncompensated motor.<br />
Compensation capacitors however, constitute a 3-phase “wattless” load for this<br />
decaying emf, which causes capacitive currents to flow through the stator windings.<br />
These stator currents will produce a rotating magnetic field in the rotor which acts<br />
exactly along the same axis and in the same direction as that <strong>of</strong> the decaying<br />
magnetic field.<br />
The rotor flux consequently increases; the stator currents increase; and the voltage<br />
at the terminals <strong>of</strong> the motor increases; sometimes to dangerously-high levels. This<br />
phenomenon is known as self-excitation and is one reason why AC generators<br />
are not normally operated at leading power factors, i.e. there is a tendency to<br />
spontaneously (and uncontrollably) self excite.<br />
Notes:<br />
1. The characteristics <strong>of</strong> a motor being driven by the inertia <strong>of</strong> the load are not<br />
rigorously identical to its no-load characteristics. This assumption, however, is<br />
sufficiently accurate for practical purposes.<br />
2. With the motor acting as a generator, the currents circulating are largely reactive,<br />
so that the braking (retarding) effect on the motor is mainly due only to the load<br />
represented by the cooling fan in the motor.<br />
3. The (almost 90° lagging) current taken from the supply in normal circumstances by<br />
the unloaded motor, and the (almost 90° leading) current supplied to the capacitors<br />
by the motor acting as a generator, both have the same phase relationship<br />
to the terminalvoltage. It is for this reason that the two characteristics may be<br />
superimposed on the graph.<br />
In order to avoid self-excitation as described above, the kvar rating <strong>of</strong> the capacitor<br />
bank must be limited to the following maximum value:<br />
Qc y 0.9 x Io x Un x 3 where Io = the no-load current <strong>of</strong> the motor and Un =<br />
phase-to-phase nominal voltage <strong>of</strong> the motor in kV. Figure L24 previous page gives<br />
appropriate values <strong>of</strong> Qc corresponding to this criterion.<br />
Example<br />
A 75 kW, 3,000 rpm, 400 V, 3-phase motor may have a capacitor bank no larger<br />
than 17 kvar according to Figure L24. The table values are, in general, too small to<br />
adequately compensate the motor to the level <strong>of</strong> cos ϕ normally required. Additional<br />
compensation can, however, be applied to the system, for example an overall bank,<br />
installed for global compensation <strong>of</strong> a number <strong>of</strong> smaller appliances.<br />
High-inertia motors and/or loads<br />
In any <strong>installation</strong> where high-inertia motor driven loads exist, the circuit-breakers or<br />
contactors controlling such motors should, in the event <strong>of</strong> total loss <strong>of</strong> power supply,<br />
be rapidly tripped.<br />
If this precaution is not taken, then self excitation to very high voltages is likely to<br />
occur, since all other banks <strong>of</strong> capacitors in the <strong>installation</strong> will effectively be in<br />
parallel with those <strong>of</strong> the high-inertia motors.<br />
The protection scheme for these motors should therefore include an overvoltage<br />
tripping relay, together with reverse-power checking contacts (the motor will feed<br />
power to the rest <strong>of</strong> the <strong>installation</strong>, until the stored inertial energy is dissipated).<br />
If the capacitor bank associated with a high inertia motor is larger than that<br />
recommended in Figure L24, then it should be separately controlled by a circuitbreaker<br />
or contactor, which trips simultaneously with the main motor-controlling<br />
circuit-breaker or contactor, as shown in Figure L26.<br />
Closing <strong>of</strong> the main contactor is commonly subject to the capacitor contactor being<br />
previously closed.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
L19<br />
© Schneider Electric - all rights reserved
L20<br />
© Schneider Electric - all rights reserved<br />
L - Power factor correction and<br />
harmonic filtering<br />
Installation before P.F. correction<br />
� � � (1)<br />
kVA=kW+kvar b kvarh are billed heavily above the declared<br />
level<br />
b Apparent power kVA is significantly greater<br />
� � �<br />
kVA=kW+kvar<br />
kVA than the kW demand<br />
kVA<br />
kW kvar<br />
b The corresponding excess current causes<br />
losses (kWh) which are billed<br />
b The <strong>installation</strong> must be over-dimensioned<br />
kW<br />
630 kVA<br />
400 V<br />
cos ϕ = 0.75<br />
workshop<br />
Characteristics <strong>of</strong> the <strong>installation</strong><br />
500 kW cos ϕ = 0.75<br />
b Transformer is overloaded<br />
b The power demand is<br />
S = P = 500 = 665 kVA<br />
cos ϕ 0.75<br />
S = apparent power<br />
b The current flowing into the <strong>installation</strong><br />
downstream <strong>of</strong> the circuit breaker is<br />
I = P = 960 A<br />
3U cos ϕ<br />
b Losses in cables are calculated as a<br />
function <strong>of</strong> the current squared: 960 2<br />
P=I 2 R<br />
cos ϕ = 0.75<br />
b Reactive energy is supplied through the<br />
transformer and via the <strong>installation</strong> wiring<br />
b The transformer, circuit breaker, and cables<br />
must be over-dimensioned<br />
8 Example <strong>of</strong> an <strong>installation</strong><br />
before and after power-factor<br />
correction<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Installation after P.F. correction<br />
630 kVA<br />
400 V<br />
cos ϕ = 0.75<br />
workshop<br />
Fig. K27 : Technical-economic comparison <strong>of</strong> an <strong>installation</strong> before and after power-factor correction<br />
(1) The arrows denote vector quantities.<br />
(2) Particularly in the pre-corrected case.<br />
b The consumption <strong>of</strong> kvarh is<br />
v Eliminated, or<br />
v Reduced, according to the cos ϕ required<br />
b The tariff penalties<br />
v For reactive energy where applicable<br />
v For the entire bill in some cases are<br />
eliminated<br />
b The fixed charge based on kVA demand is<br />
adjusted to be close to the active power kW<br />
demand<br />
Characteristics <strong>of</strong> the <strong>installation</strong><br />
500 kW cos ϕ = 0.928<br />
b Transformer no longer overloaded<br />
b The power-demand is 539 kVA<br />
b There is 14% spare-transformer capacity<br />
available<br />
b The current flowing into the <strong>installation</strong><br />
through the circuit breaker is 778 A<br />
250 kvar<br />
b The losses in the cables are<br />
reduced to 7782 = 65% <strong>of</strong> the former value,<br />
960 2<br />
thereby economizing in kWh consumed<br />
cos ϕ = 0.928<br />
b Reactive energy is supplied by the capacitor<br />
bank<br />
Capacitor bank rating is 250 kvar<br />
in 5 automatically-controlled steps <strong>of</strong> 50 kvar.<br />
Note: In fact, the cos ϕ <strong>of</strong> the workshop remains at 0.75 but cos ϕ for all the<br />
<strong>installation</strong> upstream <strong>of</strong> the capacitor bank to the transformer LV terminals<br />
is 0.928.<br />
As mentioned in Sub-clause 6.2 the cos ϕ at the HV side <strong>of</strong> the transformer<br />
will be slightly lower (2) , due to the reactive power losses in the transformer.
L - Power factor correction and<br />
harmonic filtering<br />
Harmonics are taken into account mainly by<br />
oversizing capacitors and including harmonicsuppression<br />
reactors in series with them<br />
Harmonic<br />
generator<br />
Ihar<br />
Filter<br />
Fig. L28 : Operation principle <strong>of</strong> passive filter<br />
9 The effects <strong>of</strong> harmonics<br />
9.1 Problems arising from power-system harmonics<br />
Equipment which uses power electronics components (variable-speed motor<br />
controllers, thyristor-controlled rectifiers, etc.) have considerably increased the<br />
problems caused by harmonics in power supply systems.<br />
Harmonics have existed from the earliest days <strong>of</strong> the industry and were (and still<br />
are) caused by the non-linear magnetizing impedances <strong>of</strong> transformers, reactors,<br />
fluorescent lamp ballasts, etc.<br />
Harmonics on symmetrical 3-phase power systems are generally odd-numbered: 3 rd ,<br />
5 th , 7 th , 9 th ..., and the magnitude decreases as the order <strong>of</strong> the harmonic increases.<br />
A number <strong>of</strong> features may be used in various ways to reduce specific harmonics to<br />
negligible values - total elimination is not possible. In this section, practical means <strong>of</strong><br />
reducing the influence <strong>of</strong> harmonics are recommended, with particular reference to<br />
capacitor banks.<br />
Capacitors are especially sensitive to harmonic components <strong>of</strong> the supply voltage<br />
due to the fact that capacitive reactance decreases as the frequency increases.<br />
In practice, this means that a relatively small percentage <strong>of</strong> harmonic voltage can<br />
cause a significant current to flow in the capacitor circuit.<br />
The presence <strong>of</strong> harmonic components causes the (normally sinusoidal) wave form<br />
<strong>of</strong> voltage or current to be distorted; the greater the harmonic content, the greater the<br />
degree <strong>of</strong> distortion.<br />
If the natural frequency <strong>of</strong> the capacitor bank/ power-system reactance combination<br />
is close to a particular harmonic, then partial resonance will occur, with amplified<br />
values <strong>of</strong> voltage and current at the harmonic frequency concerned. In this particular<br />
case, the elevated current will cause overheating <strong>of</strong> the capacitor, with degradation<br />
<strong>of</strong> the dielectric, which may result in its eventual failure.<br />
Several solutions to these problems are available. This can be accomplished by<br />
b Shunt connected harmonic filter and/or harmonic-suppression reactors or<br />
b Active power filters or<br />
b Hybrid filters<br />
9.2 Possible solutions<br />
Passive filter (see Fig. L28)<br />
Countering the effects <strong>of</strong> harmonics<br />
The presence <strong>of</strong> harmonics in the supply voltage results in abnormally high current<br />
levels through the capacitors. An allowance is made for this by <strong>design</strong>ing for an r.m.s.<br />
value <strong>of</strong> current equal to 1.3 times the nominal rated current. All series elements,<br />
such as connections, fuses, switches, etc., associated with the capacitors are<br />
similarly oversized, between 1.3 to 1.5 times nominal rating.<br />
Harmonic distortion <strong>of</strong> the voltage wave frequently produces a “peaky” wave form,<br />
in which the peak value <strong>of</strong> the normal sinusoidal wave is increased. This possibility,<br />
together with other overvoltage conditions likely to occur when countering the effects<br />
<strong>of</strong> resonance, as described below, are taken into account by increasing the insulation<br />
level above that <strong>of</strong> “standard” capacitors. In many instances, these two counter<br />
measures are all that is necessary to achieve satisfactory operation.<br />
Countering the effects <strong>of</strong> resonance<br />
Capacitors are linear reactive devices, and consequently do not generate harmonics.<br />
The <strong>installation</strong> <strong>of</strong> capacitors in a power system (in which the impedances are<br />
predominantly inductive) can, however, result in total or partial resonance occurring<br />
at one <strong>of</strong> the harmonic frequencies.<br />
The harmonic order ho <strong>of</strong> the natural resonant frequency between the system<br />
inductance and the capacitor bank is given by<br />
h o = Ssc<br />
Q<br />
where<br />
Ssc = the level <strong>of</strong> system short-circuit kVA at the point <strong>of</strong> connection <strong>of</strong> the capacitor<br />
Q = capacitor bank rating in kvar; and ho = the harmonic order <strong>of</strong> the natural<br />
frequency fo i.e. fo<br />
for a 50 Hz system, or fo<br />
for a 60 Hz system.<br />
50<br />
60<br />
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L22<br />
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L - Power factor correction and<br />
harmonic filtering<br />
Harmonic<br />
generator<br />
Fig. L29 : Operation principle <strong>of</strong> active filter<br />
Ihar<br />
Ihar<br />
Harmonic<br />
generator<br />
Active<br />
filter<br />
Active<br />
filter<br />
Iact<br />
Hybride filter<br />
Fig. L30 : Operation principle <strong>of</strong> hybrid filter<br />
Is<br />
Iact<br />
Linear<br />
load<br />
Is<br />
Linear<br />
load<br />
9 The effects <strong>of</strong> harmonics<br />
For example: h o = Ssc<br />
Q may give a value for h o <strong>of</strong> 2.93 which shows that the<br />
natural frequency <strong>of</strong> the capacitor/system-inductance combination is close to the 3rd harmonic frequency <strong>of</strong> the system.<br />
f<br />
From ho<br />
= o it can be seen that fo = 50 ho = 50 x 2.93 = 146.5 Hz<br />
50<br />
The closer a natural frequency approaches one <strong>of</strong> the harmonics present on the<br />
system, the greater will be the (undesirable) effect. In the above example, strong<br />
resonant conditions with the 3rd harmonic component <strong>of</strong> a distorted wave would<br />
certainly occur.<br />
In such cases, steps are taken to change the natural frequency to a value which will<br />
not resonate with any <strong>of</strong> the harmonics known to be present. This is achieved by the<br />
addition <strong>of</strong> a harmonic-suppression inductor connected in series with the capacitor<br />
bank.<br />
On 50 Hz systems, these reactors are <strong>of</strong>ten adjusted to bring the resonant frequency<br />
<strong>of</strong> the combination, i.e. the capacitor bank + reactors to 190 Hz. The reactors are<br />
adjusted to 228 Hz for a 60 Hz system. These frequencies correspond to a value<br />
for h o <strong>of</strong> 3.8 for a 50 Hz system, i.e. approximately mid-way between the 3 rd and 5 th<br />
harmonics.<br />
In this arrangement, the presence <strong>of</strong> the reactor increases the fundamental<br />
frequency (50 Hz or 60 Hz) current by a small amount (7-8%) and therefore the<br />
voltage across the capacitor in the same proportion.<br />
This feature is taken into account, for example, by using capacitors which are<br />
<strong>design</strong>ed for 440 V operation on 400 V systems.<br />
Active filter (see Fig. L29)<br />
Active filters are based on power electronic technology. They are generally installed<br />
in parallel with the non linear load.<br />
Active filters analyse the harmonics drawn by the load and then inject the same<br />
harmonic current to the load with the appropriate phase. As a result, the harmonic<br />
currents are totally neutralised at the point considered. This means they no longer<br />
flow upstream and are no longer supplied by the source.<br />
A main advantage <strong>of</strong> active conditioners is that they continue to guarantee efficient<br />
harmonic compensation even when changes are made to the <strong>installation</strong>. They are<br />
also exceptionally easy to use as they feature:<br />
b Auto-configuration to harmonic loads whatever their order <strong>of</strong> magnitude<br />
b Elimination <strong>of</strong> overload risks<br />
b Compatibility with <strong>electrical</strong> generator sets<br />
b Connection to any point <strong>of</strong> the <strong>electrical</strong> network<br />
b Several conditioners can be used in the same <strong>installation</strong> to increase depollution<br />
efficiency (for example when a new machine is installed)<br />
Active filters may provide also power factor correction.<br />
Hybrid filter (see Fig. L30)<br />
This type <strong>of</strong> filter combines advantages <strong>of</strong> passive and active filter. One frequency<br />
can be filtered by passive filter and all the other frequencies are filtered by active<br />
filter.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
9 The effects <strong>of</strong> harmonics<br />
9.3 Choosing the optimum solution<br />
Figure L31 below shows the criteria that can be taken into account to select the<br />
most suitable technology depending on the application.<br />
Passive filter Active filter Hybrid filter<br />
Applications Industrial Tertiary Industrial<br />
… with total power <strong>of</strong> non greater than lower than greater than<br />
linear loads (variable speed 200 kVA 200 kVA 200 kVA<br />
drive, UPS, rectifier…)<br />
Power factor correction No<br />
Necessity <strong>of</strong> reducing the<br />
harmonic distorsion in<br />
voltage for sensitive loads<br />
Necessity <strong>of</strong> reducing<br />
the harmonic distorsion<br />
in current to avoid cable<br />
overload<br />
Necessity <strong>of</strong> being in No<br />
accordance with strict<br />
limits <strong>of</strong> harmonic<br />
rejected<br />
Fig. L31 : Selection <strong>of</strong> the most suitable technology depending on the application<br />
For passive filter, a choice is made from the following parameters:<br />
b Gh = the sum <strong>of</strong> the kVA ratings <strong>of</strong> all harmonic-generating devices (static<br />
converters, inverters, speed controllers, etc.) connected to the busbars from which<br />
the capacitor bank is supplied. If the ratings <strong>of</strong> some <strong>of</strong> these devices are quoted in<br />
kW only, assume an average power factor <strong>of</strong> 0.7 to obtain the kVA ratings<br />
b Ssc = the 3-phase short-circuit level in kVA at the terminals <strong>of</strong> the capacitor bank<br />
b Sn = the sum <strong>of</strong> the kVA ratings <strong>of</strong> all transformers supplying (i.e. directly<br />
connected to) the system level <strong>of</strong> which the busbars form a part<br />
If a number <strong>of</strong> transformers are operating in parallel, the removal from service <strong>of</strong> one<br />
or more, will significantly change the values <strong>of</strong> Ssc and Sn. From these parameters,<br />
a choice <strong>of</strong> capacitor specification which will ensure an acceptable level <strong>of</strong> operation<br />
with the system harmonic voltages and currents, can be made, by reference to<br />
Figure L32.<br />
b <strong>General</strong> rule valid for any size <strong>of</strong> transformer<br />
Ssc<br />
Gh i<br />
120<br />
Ssc Ssc<br />
i Gh i<br />
120 70<br />
Ssc<br />
Gh ><br />
70<br />
Standard capacitors Capacitor voltage rating Capacitor voltage rating<br />
increased by 10% increased by 10%<br />
(except 230 V units) + harmonic-suppression reactor<br />
b Simplified rule if transformer(s) rating Sn y 2 MVA<br />
Gh i 0.15 Sn 0.15 Sn < Gh i 0.25 Sn 0.25 Sn < Gh i 0.60 Sn Gh > 0.60 Sn<br />
Standard capacitors Capacitor voltage rating Capacitor voltage rating Filters<br />
increased by 10% increased by 10%<br />
(except 230 V units) + harmonic suppression<br />
reactor<br />
Fig. L32 : Choice <strong>of</strong> solutions for limiting harmonics associated with a LV capacitor bank supplied<br />
via transformer(s)<br />
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L - Power factor correction and<br />
harmonic filtering<br />
(1) Merlin-Gerin <strong>design</strong>ation<br />
10 Implementation <strong>of</strong> capacitor<br />
banks<br />
10.1 Capacitor elements<br />
Technology<br />
The capacitors are dry-type units (i.e. are not impregnated by liquid dielectric)<br />
comprising metallized polypropylene self-healing film in the form <strong>of</strong> a two-film roll.<br />
They are protected by a high-quality system (overpressure disconnector used with<br />
a high breaking capacity fuse) which switches <strong>of</strong>f the capacitor if an internal fault<br />
occurs.<br />
The protection scheme operates as follows:<br />
b A short-circuit through the dielectric will blow the fuse<br />
b Current levels greater than normal, but insufficient to blow the fuse sometimes<br />
occur, e.g. due to a microscopic flow in the dielectric film. Such “faults” <strong>of</strong>ten re-seal<br />
due to local heating caused by the leakage current, i.e. the units are said to be “selfhealing”<br />
b If the leakage current persists, the defect may develop into a short-circuit, and the<br />
fuse will blow<br />
b Gas produced by vaporizing <strong>of</strong> the metallisation at the faulty location will gradually<br />
build up a pressure within the plastic container, and will eventually operate a<br />
pressure-sensitive device to short-circuit the unit, thereby causing the fuse to blow<br />
Capacitors are made <strong>of</strong> insulating material providing them with double insulation and<br />
avoiding the need for a ground connection (see Fig. L33).<br />
a)<br />
b)<br />
HRC fuse<br />
Discharge<br />
resistor<br />
Metallic<br />
disc<br />
Overpressure disconnect<br />
device<br />
Electrical characteristics<br />
Standard IEC 60439-1, NFC 54-104, VDE 0560 CSA<br />
Standards, UL tests<br />
Operating range Rated voltage 400 V<br />
Rated frequency 50 Hz<br />
Capacitance tolerance - 5% to + 10%<br />
Temperature range Maximum temperature 55 °C<br />
(up to 65 kvar) Average temperature over<br />
24 h<br />
45 °C<br />
Average annual<br />
temperature<br />
35 °C<br />
Minimum temperature - 25 °C<br />
Insulation level 50 Hz 1 min withstand voltage : 6 kV<br />
1.2/50 μs impulse withstand voltage : 25 kV<br />
Permissible current overload Classic range (1) Comfort range (1)<br />
30% 50%<br />
Permissible voltage overload 10% 20%<br />
Fig. L33 : Capacitor element, (a) cross-section, (b) <strong>electrical</strong> characteristics<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
L - Power factor correction and<br />
harmonic filtering<br />
(1) Merlin-Gerin <strong>design</strong>ation<br />
(2) Harmony capacitor banks are equipped with a harmonic<br />
suppression reactor.<br />
10 Implementation <strong>of</strong> capacitor<br />
banks<br />
10.2 Choice <strong>of</strong> protection, control devices and<br />
connecting cables<br />
The choice <strong>of</strong> upstream cables and protection and control devices depends on the<br />
current loading.<br />
For capacitors, the current is a function <strong>of</strong>:<br />
b The applied voltage and its harmonics<br />
b The capacitance value<br />
The nominal current In <strong>of</strong> a 3-phase capacitor bank is equal to:<br />
In =<br />
Un 3<br />
Q<br />
with:<br />
v Q: kvar rating<br />
v Un: Phase-to-phase voltage (kV)<br />
The permitted range <strong>of</strong> applied voltage at fundamental frequency, plus harmonic<br />
components, together with manufacturing tolerances <strong>of</strong> actual capacitance (for a<br />
declared nominal value) can result in a 50% increase above the calculated value <strong>of</strong><br />
current. Approximately 30% <strong>of</strong> this increase is due to the voltage increases, while a<br />
further 15% is due to the range <strong>of</strong> manufacturing tolerances, so that<br />
1.3 x 1.15 = 1.5<br />
All components carrying the capacitor current therefore, must be adequate to cover<br />
this “worst-case” condition, in an ambient temperature <strong>of</strong> 50 °C maximum. In the<br />
case where temperatures higher than 50 °C occur in enclosures, etc. derating <strong>of</strong> the<br />
components will be necessary.<br />
Protection<br />
The size <strong>of</strong> the circuit-breaker can be chosen in order to allow the setting <strong>of</strong> long<br />
time delay at:<br />
b 1.36 x In for Classic range (1)<br />
b 1.50 x In for Comfort range (1)<br />
b 1.12 x In for Harmony range (1) (tuned at 2.7 f) (2)<br />
b 1.19 x In for Harmony range (1) (tuned at 3.8 f)<br />
b 1.31 x In for Harmony range (1) (tuned at 4.3 f)<br />
Short time delay setting (short-circuit protection) must be insensitive to inrush<br />
current. The setting will be 10 x In for Classic, Comfort and Harmony range (1) .<br />
Example 1<br />
50 kvar – 400V – 50 Hz – Classic range<br />
50, 000<br />
In = = 72 A<br />
( 400 x 1.732)<br />
Long time delay setting: 1.36 x 72 = 98 A<br />
Short time delay setting: 10 x In = 720 A<br />
Example 2<br />
50 kvar – 400V – 50 Hz – Harmony range (tuned at 4.3 f)<br />
In = 72 A<br />
Long time delay setting: 1.31 x 72 = 94 A<br />
Short time delay setting: 10 x In = 720 A<br />
Upstream cables<br />
Figure L34 next page gives the minimum cross section area <strong>of</strong> the upstream cable<br />
for Rectiphase capacitors.<br />
Cables for control<br />
The minimum cross section area <strong>of</strong> these cables will be 1.5 mm 2 for 230 V.<br />
For the secondary side <strong>of</strong> the transformer, the recommended cross section<br />
area is u 2.5 mm 2 .<br />
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L - Power factor correction and<br />
harmonic filtering<br />
(1) Minimum cross-section not allowing for any correction<br />
factors (<strong>installation</strong> mode, temperature, etc.). The calculations<br />
were made for single-pole cables laid in open air at 30 °C.<br />
10 Implementation <strong>of</strong> capacitor<br />
banks<br />
Fig L34 : Cross-section <strong>of</strong> cables connecting medium and high power capacitor banks (1)<br />
Voltage transients<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Bank power Copper Aluminium<br />
(kvar) cross- section cross- section<br />
230 V 400 V (mm 2 ) (mm 2 )<br />
5 10 2.5 16<br />
10 20 4 16<br />
15 30 6 16<br />
20 40 10 16<br />
25 50 16 25<br />
30 60 25 35<br />
40 80 35 50<br />
50 100 50 70<br />
60 120 70 95<br />
70 140 95 120<br />
90-100 180 120 185<br />
200 150 240<br />
120 240 185 2 x 95<br />
150 250 240 2 x 120<br />
300 2 x 95 2 x 150<br />
180-210 360 2 x 120 2 x 185<br />
245 420 2 x 150 2 x 240<br />
280 480 2 x 185 2 x 300<br />
315 540 2 x 240 3 x 185<br />
350 600 2 x 300 3 x 240<br />
385 660 3 x 150 3 x 240<br />
420 720 3 x 185 3 x 300<br />
High-frequency voltage and current transients occur when switching a capacitor<br />
bank into service. The maximum voltage peak does not exceed (in the absence <strong>of</strong><br />
harmonics) twice the peak value <strong>of</strong> the rated voltage when switching uncharged<br />
capacitors.<br />
In the case <strong>of</strong> a capacitor being already charged at the instant <strong>of</strong> switch closure,<br />
however, the voltage transient can reach a maximum value approaching 3 times the<br />
normal rated peak value.<br />
This maximum condition occurs only if:<br />
b The existing voltage at the capacitor is equal to the peak value <strong>of</strong> rated voltage,<br />
and<br />
b The switch contacts close at the instant <strong>of</strong> peak supply voltage, and<br />
b The polarity <strong>of</strong> the power-supply voltage is opposite to that <strong>of</strong> the charged<br />
capacitor<br />
In such a situation, the current transient will be at its maximum possible value, viz:<br />
Twice that <strong>of</strong> its maximum when closing on to an initially uncharged capacitor, as<br />
previously noted.<br />
For any other values <strong>of</strong> voltage and polarity on the pre-charged capacitor, the<br />
transient peaks <strong>of</strong> voltage and current will be less than those mentioned above.<br />
In the particular case <strong>of</strong> peak rated voltage on the capacitor having the same polarity<br />
as that <strong>of</strong> the supply voltage, and closing the switch at the instant <strong>of</strong> supply-voltage<br />
peak, there would be no voltage or current transients.<br />
Where automatic switching <strong>of</strong> stepped banks <strong>of</strong> capacitors is considered, therefore,<br />
care must be taken to ensure that a section <strong>of</strong> capacitors about to be energized is<br />
fully discharged.<br />
The discharge delay time may be shortened, if necessary, by using discharge<br />
resistors <strong>of</strong> a lower resistance value.
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
<strong>Chapter</strong> M<br />
Harmonic management<br />
Contents<br />
The problem: why is it necessary to detect M2<br />
and eliminate harmonics?<br />
Standards M3<br />
<strong>General</strong> M4<br />
Main effects <strong>of</strong> harmonics in <strong>installation</strong>s M6<br />
4.1 Resonance M6<br />
4.2 Increased losses M6<br />
4.3 Overloads on equipment M7<br />
4.4 Disturbances affecting sensitive loads M9<br />
4.5 Economic impact M10<br />
Essential indicators <strong>of</strong> harmonic distortion M<br />
and measurement principles<br />
5.1 Power factor M11<br />
5.2 Crest factor M11<br />
5.3 Power values and harmonics M11<br />
5.4 Harmonic spectrum and harmonic distortion M12<br />
5.5 Total harmonic distortion (THD) M12<br />
5.6 Usefulness <strong>of</strong> the various indicators M13<br />
Measuring the indicators M 4<br />
6.1 Devices used to measure the indicators M14<br />
6.2 Procedures for harmonic analysis <strong>of</strong> distribution networks M14<br />
6.3 Keeping a close eye on harmonics M15<br />
Detection devices M 6<br />
Solutions to attenuate harmonics M 7<br />
8.1 Basic solutions M17<br />
8.2 Harmonic filtering M18<br />
8.3 The method M20<br />
8.4 Specific products M20<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
M<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
M2<br />
M - Harmonic management<br />
The problem: why is it necessary<br />
to detect and eliminate harmonics?<br />
Disturbances caused by harmonics<br />
Harmonics flowing in distribution networks downgrade the quality <strong>of</strong> <strong>electrical</strong> power.<br />
This can have a number <strong>of</strong> negative effects:<br />
b Overloads on distribution networks due to the increase in rms current<br />
b Overloads in neutral conductors due to the cumulative increase in third-order<br />
harmonics created by single-phase loads<br />
b Overloads, vibration and premature ageing <strong>of</strong> generators, transformers and motors<br />
as well as increased transformer hum<br />
b Overloads and premature ageing <strong>of</strong> power-factor correction capacitors<br />
b Distortion <strong>of</strong> the supply voltage that can disturb sensitive loads<br />
b Disturbances in communication networks and on telephone lines<br />
Economic impact <strong>of</strong> disturbances<br />
Harmonics have a major economic impact:<br />
b Premature ageing <strong>of</strong> equipment means it must be replaced sooner unless<br />
oversized right from the start<br />
b Overloads on the distribution network can require higher subscribed power levels<br />
and increase losses<br />
b Distortion <strong>of</strong> current waveforms provokes nuisance tripping that can stop<br />
production<br />
Increasingly serious consequences<br />
Only ten years ago, harmonics were not yet considered a real problem because<br />
their effects on distribution networks were generally minor. However, the massive<br />
introduction <strong>of</strong> power electronics in equipment has made the phenomenon far more<br />
serious in all sectors <strong>of</strong> economic activity.<br />
In addition, the equipment causing the harmonics is <strong>of</strong>ten vital to the company or<br />
organisation.<br />
Which harmonics must be measured and eliminated?<br />
The most frequently encountered harmonics in three-phase distribution networks<br />
are the odd orders. Harmonic amplitudes normally decrease as the frequency<br />
increases. Above order 50, harmonics are negligible and measurements are no<br />
longer meaningful. Sufficiently accurate measurements are obtained by measuring<br />
harmonics up to order 30.<br />
Utilities monitor harmonic orders 3, 5, 7, 11 and 13. <strong>General</strong>ly speaking, harmonic<br />
conditioning <strong>of</strong> the lowest orders (up to 13) is sufficient. More comprehensive<br />
conditioning takes into account harmonic orders up to 25.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
M - Harmonic management<br />
Fig. M1 : Maximum permissible harmonic levels<br />
2 Standards<br />
Harmonic emissions are subject to various standards and regulations:<br />
b Compatibility standards for distribution networks<br />
b Emissions standards applying to the equipment causing harmonics<br />
b Recommendations issued by utilities and applicable to <strong>installation</strong>s<br />
In view <strong>of</strong> rapidly attenuating the effects <strong>of</strong> harmonics, a triple system <strong>of</strong> standards<br />
and regulations is currently in force based on the documents listed below.<br />
Standards governing compatibility between distribution networks and<br />
products<br />
These standards determine the necessary compatibility between distribution<br />
networks and products:<br />
b The harmonics caused by a device must not disturb the distribution network<br />
beyond certain limits<br />
b Each device must be capable <strong>of</strong> operating normally in the presence <strong>of</strong><br />
disturbances up to specific levels<br />
b Standard IEC 61000-2-2 for public low-voltage power supply systems<br />
b Standard IEC 61000-2-4 for LV and MV industrial <strong>installation</strong>s<br />
Standards governing the quality <strong>of</strong> distribution networks<br />
b Standard EN 50160 stipulates the characteristics <strong>of</strong> electricity supplied by public<br />
distribution networks<br />
b Standard IEEE 519 presents a joint approach between Utilities and customers<br />
to limit the impact <strong>of</strong> non-linear loads. What is more, Utilities encourage preventive<br />
action in view <strong>of</strong> reducing the deterioration <strong>of</strong> power quality, temperature rise and the<br />
reduction <strong>of</strong> power factor. They will be increasingly inclined to charge customers for<br />
major sources <strong>of</strong> harmonics<br />
Standards governing equipment<br />
b Standard IEC 61000-3-2 or EN 61000-3-2 for low-voltage equipment with rated<br />
current under 16 A<br />
b Standard IEC 61000-3-12 for low-voltage equipment with rated current higher than<br />
16 A and lower than 75 A<br />
Maximum permissible harmonic levels<br />
International studies have collected data resulting in an estimation <strong>of</strong> typical<br />
harmonic contents <strong>of</strong>ten encountered in <strong>electrical</strong> distribution networks. Figure M1<br />
presents the levels that, in the opinion <strong>of</strong> many utilities, should not be exceeded.<br />
Odd harmonic orders Odd harmonic orders Even harmonic orders<br />
non-multiples <strong>of</strong> multiples <strong>of</strong><br />
Order h LV MV EMV Order h LV MV EMV Order h LV MV EMV<br />
5 6 6 2 3 5 2.5 1.5 2 2 1.5 1.5<br />
7 5 5 2 9 1.5 1.5 1 4 1 1 1<br />
11 3.5 3.5 1.5 15 0.3 0.3 0.3 6 0.5 0.5 0.5<br />
13 3 3 1.5 21 0.2 0.2 0.2 8 0.5 0.2 0.2<br />
17 2 2 1 > 21 0.2 0.2 0.2 10 0.5 0.2 0.2<br />
19 1.5 1.5 1 12 0.2 0.2 0.2<br />
23 1.5 1 0.7 > 12 0.2 0.2 0.2<br />
25 1.5 1 0.7<br />
> 25 0.2 0.2 0.1<br />
+ 25/h + 25/h + 25/h<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
M<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
M<br />
M - Harmonic management<br />
3 <strong>General</strong><br />
The presence <strong>of</strong> harmonics indicates a distorted current or voltage wave. The<br />
distortion <strong>of</strong> the current or voltage wave means that the distribution <strong>of</strong> <strong>electrical</strong><br />
energy is disturbed and power quality is not optimum.<br />
Harmonic currents are caused by non-linear loads connected to the distribution<br />
network. The flow <strong>of</strong> harmonic currents causes harmonic voltages via distributionnetwork<br />
impedances and consequently distortion <strong>of</strong> the supply voltage.<br />
Origin <strong>of</strong> harmonics<br />
Devices and systems that cause harmonics are present in all sectors, i.e. industrial,<br />
commercial and residential. Harmonics are caused by non-linear loads (i.e. loads<br />
that draw current with a waveform that is not the same as that <strong>of</strong> the supply voltage).<br />
Examples <strong>of</strong> non-linear loads are:<br />
b Industrial equipment (welding machines, arc furnaces, induction furnaces,<br />
rectifiers)<br />
b Variable-speed drives for asynchronous or DC motors<br />
b UPSs<br />
b Office equipment (computers, photocopy machines, fax machines, etc.)<br />
b Home appliances (television sets, micro-wave ovens, fluorescent lighting)<br />
b Certain devices involving magnetic saturation (transformers)<br />
Disturbances caused by non-linear loads: harmonic current and voltage<br />
Non-linear loads draw harmonic currents that flow in the distribution network.<br />
Harmonic voltages are caused by the flow <strong>of</strong> harmonic currents through the<br />
impedances <strong>of</strong> the supply circuits (transformer and distribution network for situations<br />
similar to that shown in Figure M2).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
A<br />
Z h B<br />
I h<br />
Non-linear<br />
load<br />
Fig. M2 : Single-line diagram showing the impedance <strong>of</strong> the supply circuit for a harmonic <strong>of</strong> order h<br />
The reactance <strong>of</strong> a conductor increases as a function <strong>of</strong> the frequency <strong>of</strong> the current<br />
flowing through the conductor. For each harmonic current (order h), there is therefore<br />
an impedance Zh in the supply circuit.<br />
When the harmonic current <strong>of</strong> order h flows through impedance Zh, it creates a<br />
harmonic voltage Uh, where Uh = Zh x Ih (Ohm law). The voltage at point B is<br />
therefore distorted. All devices supplied via point B receive a distorted voltage.<br />
For a given harmonic current, the distortion is proportional to the impedance in the<br />
distribution network.<br />
Flow <strong>of</strong> harmonic currents in distribution networks<br />
The non-linear loads can be considered to reinject the harmonic currents upstream<br />
into the distribution network, toward the source.<br />
Figures M3 and M next page show an <strong>installation</strong> disturbed by harmonics. Figure<br />
M3 shows the flow <strong>of</strong> the current at 50 Hz in the <strong>installation</strong> and Figure M4 shows<br />
the harmonic current (order h).
M - Harmonic management<br />
3 <strong>General</strong><br />
Fig. M3 : Installation supplying a non-linear load, where only the phenomena concerning the<br />
50 Hz frequency (fundamental frequency) are shown<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Z h<br />
Z l<br />
I 50 Hz<br />
Non-linear<br />
load<br />
Fig. M4 : Same <strong>installation</strong>, where only the phenomena concerning the frequency <strong>of</strong> harmonic<br />
order h are shown<br />
V h<br />
I h<br />
Non-linear<br />
load<br />
V h = Harmonic voltage<br />
= Z h x I h<br />
Supply <strong>of</strong> the non-linear load creates the flow <strong>of</strong> a current I 50Hz (shown in<br />
figure M3), to which is added each <strong>of</strong> the harmonic currents Ih (shown in figure M4),<br />
corresponding to each harmonic order h.<br />
Still considering that the loads reinject harmonic current upstream into the<br />
distribution network, it is possible to create a diagram showing the harmonic currents<br />
in the network (see Fig. M ).<br />
Backup power<br />
supply<br />
G<br />
Power-factor<br />
correction<br />
MV/LV<br />
Harmonic<br />
disturbances to<br />
distribution network<br />
and other users<br />
A<br />
Ih a<br />
Ih b<br />
Ih d<br />
Ih e<br />
(do not create<br />
harmonics)<br />
Rectifier<br />
Arc furnace<br />
Welding machine<br />
Variable-speed drive<br />
Fluorescent or<br />
discharge lamps<br />
Devices drawing rectified<br />
current (televisions,<br />
computer hardware, etc.)<br />
Linear loads<br />
Note in the diagram that though certain loads create harmonic currents in the distribution<br />
network, other loads can absorb the harmonic currents.<br />
Fig. M5 : Flow <strong>of</strong> harmonic currents in a distribution network<br />
Harmonics have major economic effects in <strong>installation</strong>s:<br />
b Increases in energy costs<br />
b Premature ageing <strong>of</strong> equipment<br />
b Production losses<br />
M<br />
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© Schneider Electric - all rights reserved<br />
M<br />
M - Harmonic management<br />
I h<br />
Non-linear<br />
load<br />
C<br />
Capacitor<br />
bank<br />
Fig. M6 : Diagram <strong>of</strong> an <strong>installation</strong><br />
Ls C R Ih<br />
Z<br />
Linear<br />
load<br />
Fig. M7 : Equivalent diagram <strong>of</strong> the <strong>installation</strong> shown in<br />
Figure M6<br />
4 Main effects <strong>of</strong> harmonics in<br />
<strong>installation</strong>s<br />
4.1 Resonance<br />
The simultaneous use <strong>of</strong> capacitive and inductive devices in distribution networks<br />
results in parallel or series resonance manifested by very high or very low<br />
impedance values respectively. The variations in impedance modify the current and<br />
voltage in the distribution network. Here, only parallel resonance phenomena, the<br />
most common, will be discussed.<br />
Consider the following simplified diagram (see Fig. M ) representing an <strong>installation</strong><br />
made up <strong>of</strong>:<br />
b A supply transformer<br />
b Linear loads<br />
b Non-linear loads drawing harmonic currents<br />
b Power factor correction capacitors<br />
For harmonic analysis, the equivalent diagram (see Fig. M7) is shown below.<br />
Impedance Z is calculated by:<br />
jLs<br />
Z =<br />
1-LsC 2<br />
ω<br />
ω<br />
neglecting R and where:<br />
Ls = Supply inductance (upstream network + transformer + line)<br />
C = Capacitance <strong>of</strong> the power factor correction capacitors<br />
R = Resistance <strong>of</strong> the linear loads<br />
Ih = Harmonic current<br />
Resonance occurs when the denominator 1-LsCw 2 tends toward zero. The<br />
corresponding frequency is called the resonance frequency <strong>of</strong> the circuit. At that<br />
frequency, impedance is at its maximum and high amounts <strong>of</strong> harmonic voltages<br />
appear with the resulting major distortion in the voltage. The voltage distortion is<br />
accompanied, in the Ls+C circuit, by the flow <strong>of</strong> harmonic currents greater than<br />
those drawn by the loads.<br />
The distribution network and the power factor correction capacitors are subjected to<br />
high harmonic currents and the resulting risk <strong>of</strong> overloads. To avoid resonance, antiharmonic<br />
coils can be installed in series with the capacitors.<br />
4.2 Increased losses<br />
Losses in conductors<br />
The active power transmitted to a load is a function <strong>of</strong> the fundamental component I1<br />
<strong>of</strong> the current.<br />
When the current drawn by the load contains harmonics, the rms value <strong>of</strong> the<br />
current, Irms, is greater than the fundamental I1.<br />
The definition <strong>of</strong> THD being:<br />
THD =<br />
2<br />
⎛ Irms⎞<br />
⎜ ⎟ −1<br />
⎝ I1<br />
⎠<br />
it may be deduced that: Irms = I1<br />
+ THD 2<br />
1<br />
Figure M8 (next page) shows, as a function <strong>of</strong> the harmonic distortion:<br />
b The increase in the rms current Irms for a load drawing a given fundamental<br />
current<br />
b The increase in Joule losses, not taking into account the skin effect<br />
(The reference point in the graph is 1 for Irms and Joules losses, the case when<br />
there are no harmonics)<br />
The harmonic currents provoke an increase in the Joule losses in all conductors in<br />
which they flow and additional temperature rise in transformers, devices, cables, etc.<br />
Losses in asynchronous machines<br />
The harmonic voltages (order h) supplied to asynchronous machines provoke in the<br />
rotor the flow <strong>of</strong> currents with frequencies higher than 50 Hz that are the cause <strong>of</strong><br />
additional losses.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
M - Harmonic management<br />
4 Main effects <strong>of</strong> harmonics in<br />
<strong>installation</strong>s<br />
2.2<br />
2<br />
1.8<br />
1.6<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0 20 40 60 80 100 120<br />
Joules losses<br />
Irms<br />
Fig. M8 : Increase in rms current and Joule losses as a function <strong>of</strong> the THD<br />
Orders <strong>of</strong> magnitude<br />
b A virtually rectangular supply voltage provokes a 20% increase in losses<br />
b A supply voltage with harmonics u5 = 8% (<strong>of</strong> U1, the fundamental voltage),<br />
u7 = 5%, u11 = 3%, u13 = 1%, i.e. total harmonic distortion THDu equal to 10%,<br />
results in additional losses <strong>of</strong> 6%<br />
Losses in transformers<br />
Harmonic currents flowing in transformers provoke an increase in the “copper”<br />
losses due to the Joule effect and increased “iron” losses due to eddy currents. The<br />
harmonic voltages are responsible for “iron” losses due to hysteresis.<br />
It is generally considered that losses in windings increase as the square <strong>of</strong> the THDi<br />
and that core losses increase linearly with the THDu.<br />
In utility-distribution transformers, where distortion levels are limited, losses increase<br />
between 10 and 15%.<br />
Losses in capacitors<br />
The harmonic voltages applied to capacitors provoke the flow <strong>of</strong> currents<br />
proportional to the frequency <strong>of</strong> the harmonics. These currents cause additional<br />
losses.<br />
Example<br />
A supply voltage has the following harmonics:<br />
Fundamental voltage U1, harmonic voltages u5 = 8% (<strong>of</strong> U1), u7 = 5%, u11 = 3%,<br />
u13 = 1%, i.e. total harmonic distortion THDu equal to 10%. The amperage <strong>of</strong> the<br />
current is multiplied by 1.19. Joule losses are multiplied by 1.19 2 , i.e. 1.4.<br />
4.3 Overloads on equipment<br />
Generators<br />
Generators supplying non-linear loads must be derated due to the additional losses<br />
caused by harmonic currents.<br />
The level <strong>of</strong> derating is approximately 10% for a generator where the overall load<br />
is made up <strong>of</strong> 30% <strong>of</strong> non-linear loads. It is therefore necessary to oversize the<br />
generator.<br />
Uninterruptible power systems (UPS)<br />
The current drawn by computer systems has a very high crest factor. A UPS sized<br />
taking into account exclusively the rms current may not be capable <strong>of</strong> supplying the<br />
necessary peak current and may be overloaded.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
THD<br />
(%)<br />
M7<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
M8<br />
M - Harmonic management<br />
(1) In fact, the current waveform is similar to a rectangular<br />
waveform. This is the case for all current rectifiers (three-phase<br />
rectifiers, induction furnaces).<br />
4 Main effects <strong>of</strong> harmonics in<br />
<strong>installation</strong>s<br />
Transformers<br />
b The curve presented below (see Fig. M9) shows the typical derating required for a<br />
transformer supplying electronic loads<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
Example<br />
If the transformer supplies an overall load comprising 40% <strong>of</strong> electronic loads, it must<br />
be derated by 40%.<br />
b Standard UTE C15-112 provides a derating factor for transformers as a function <strong>of</strong><br />
the harmonic currents.<br />
k =<br />
kVA<br />
(%)<br />
0<br />
Th = Ih<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
1<br />
⎛ 40 ⎞<br />
1.6 2<br />
1+ 0.1 ⎜ ∑h Th ⎟<br />
⎝ h= 2 ⎠<br />
I<br />
1<br />
Typical values:<br />
b Current with a rectangular waveform (1/h spectrum (1) ): k = 0.86<br />
b Frequency-converter current (THD ≈ 50%): k = 0.80<br />
Asynchronous machines<br />
Standard IEC 60892 defines a weighted harmonic factor (Harmonic voltage factor)<br />
for which the equation and maximum value are provided below.<br />
HVF =<br />
0 20 40 60 80 100<br />
Fig. M9 : Derating required for a transformer supplying electronic loads<br />
13 U h<br />
∑ 0.02<br />
2<br />
h i<br />
h= 2<br />
Example<br />
A supply voltage has a fundamental voltage U1 and harmonic voltages u3 = 2% <strong>of</strong><br />
U1, u5 = 3%, u7 = 1%. The THDu is 3.7% and the MVF is 0.018. The MVF value<br />
is very close to the maximum value above which the machine must be derated.<br />
Practically speaking, for supply to the machine, a THDu <strong>of</strong> 10% must not be<br />
exceeded.<br />
Capacitors<br />
%<br />
Electronic<br />
load<br />
According to IEC 60831-1 standard, the rms current flowing in the capacitors must<br />
not exceed 1.3 times the rated current.<br />
Using the example mentioned above, the fundamental voltage U1, harmonic voltages<br />
voltages u5 = 8% u5 (<strong>of</strong> = U1), 8% u7 (<strong>of</strong> = U1), 5%, u7 u11 = 5%, = 3%, u11 u13 = 3%, = 1%, u13 i.e. = total 1%, harmonic i.e. total harmonic<br />
distortion THDu equal to 10%, the result is Irms<br />
= 1. 19,<br />
, at the rated voltage. For a<br />
I1<br />
, at the rated voltage. For a<br />
voltage equal equal to to 1.1 1.1 times the the rated voltage, the current limit Irms<br />
= 1. 3 is reached<br />
and it is necessary to resize the capacitors.<br />
I1
M - Harmonic management<br />
I r<br />
I s<br />
I t<br />
I n<br />
Load<br />
Load<br />
Load<br />
Fig. M10 : Flow <strong>of</strong> currents in the various conductors<br />
connected to a three-phase source<br />
4 Main effects <strong>of</strong> harmonics in<br />
<strong>installation</strong>s<br />
Neutral conductors<br />
Consider a system made up <strong>of</strong> a balanced three-phase source and three identical<br />
single-phase loads connected between the phases and the neutral (see Fig. M10).<br />
Figure M11 shows an example <strong>of</strong> the currents flowing in the three phases and the<br />
resulting current in the neutral conductor.<br />
In this example, the current in the neutral conductor has an rms value that is higher<br />
than the rms value <strong>of</strong> the current in a phase by a factor equal to the square root <strong>of</strong> 3.<br />
The neutral conductor must therefore be sized accordingly.<br />
Ir<br />
Is<br />
It<br />
In<br />
(A)<br />
Fig. M11 : Example <strong>of</strong> the currents flowing in the various conductors connected to a three-phase<br />
load (In = Ir + Is + It)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
0<br />
20 40<br />
4.4 Disturbances affecting sensitive loads<br />
Effects <strong>of</strong> distortion in the supply voltage<br />
t<br />
t<br />
t<br />
t<br />
t (ms)<br />
Distortion <strong>of</strong> the supply voltage can disturb the operation <strong>of</strong> sensitive devices:<br />
b Regulation devices (temperature)<br />
b Computer hardware<br />
b Control and monitoring devices (protection relays)<br />
Distortion <strong>of</strong> telephone signals<br />
Harmonics cause disturbances in control circuits (low current levels). The level <strong>of</strong><br />
distortion depends on the distance that the power and control cables run in parallel,<br />
the distance between the cables and the frequency <strong>of</strong> the harmonics.<br />
M9<br />
© Schneider Electric - all rights reserved
M10<br />
© Schneider Electric - all rights reserved<br />
M - Harmonic management<br />
4 Main effects <strong>of</strong> harmonics in<br />
<strong>installation</strong>s<br />
4.5 Economic impact<br />
Energy losses<br />
Harmonics cause additional losses (Joule effect) in conductors and equipment.<br />
Higher subscription costs<br />
The presence <strong>of</strong> harmonic currents can require a higher subscribed power level and<br />
consequently higher costs.<br />
What is more, utilities will be increasingly inclined to charge customers for major<br />
sources <strong>of</strong> harmonics.<br />
Oversizing <strong>of</strong> equipment<br />
b Derating <strong>of</strong> power sources (generators, transformers and UPSs) means they must<br />
be oversized<br />
b Conductors must be sized taking into account the flow <strong>of</strong> harmonic currents.<br />
In addition, due the the skin effect, the resistance <strong>of</strong> these conductors increases<br />
with frequency. To avoid excessive losses due to the Joule effect, it is necessary to<br />
oversize conductors<br />
b Flow <strong>of</strong> harmonics in the neutral conductor means that it must be oversized as well<br />
Reduced service life <strong>of</strong> equipment<br />
When the level <strong>of</strong> distortion in the supply voltage approaches 10%, the duration<br />
<strong>of</strong> the service life <strong>of</strong> equipment is significantly reduced. The reduction has been<br />
estimated at:<br />
b 32.5% for single-phase machines<br />
b 18% for three-phase machines<br />
b 5% for transformers<br />
To maintain the service lives corresponding to the rated load, equipment must be<br />
oversized.<br />
Nuisance tripping and <strong>installation</strong> shutdown<br />
Circuit-breakers in the <strong>installation</strong> are subjected to current peaks caused by<br />
harmonics.<br />
These current peaks cause nuisance tripping with the resulting production losses, as<br />
well as the costs corresponding to the time required to start the <strong>installation</strong> up again.<br />
Examples<br />
Given the economic consequences for the <strong>installation</strong>s mentioned below, it was<br />
necessary to install harmonic filters.<br />
Computer centre for an insurance company<br />
In this centre, nuisance tripping <strong>of</strong> a circuit-breaker was calculated to have cost<br />
100 k€ per hour <strong>of</strong> down time.<br />
Pharmaceutical laboratory<br />
Harmonics caused the failure <strong>of</strong> a generator set and the interruption <strong>of</strong> a longduration<br />
test on a new medication. The consequences were a loss estimated at<br />
17 M€.<br />
Metallurgy factory<br />
A set <strong>of</strong> induction furnaces caused the overload and destruction <strong>of</strong> three<br />
transformers ranging from 1500 to 2500 kVA over a single year. The cost <strong>of</strong> the<br />
interruptions in production were estimated at 20 k€ per hour.<br />
Factory producing garden furniture<br />
The failure <strong>of</strong> variable-speed drives resulted in production shutdowns estimated at<br />
10 k€ per hour.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
M - Harmonic management<br />
5 Essential indicators <strong>of</strong> harmonic<br />
distortion and measurement<br />
principles<br />
A number <strong>of</strong> indicators are used to quantify and evaluate the harmonic distortion<br />
in current and voltage waveforms, namely:<br />
b Power factor<br />
b Crest factor<br />
b Distortion power<br />
b Harmonic spectrum<br />
b Harmonic-distortion values<br />
These indicators are indispensable in determining any necessary corrective action.<br />
5.1 Power factor<br />
Definition<br />
The power factor PF is the ratio between the active power P and the apparent<br />
power S.<br />
PF P<br />
=<br />
S<br />
Among electricians, there is <strong>of</strong>ten confusion with:<br />
P1<br />
cos<br />
ϕ =<br />
S1<br />
Where<br />
Where<br />
P1 = active power <strong>of</strong> the fundamental<br />
S1 = apparent power <strong>of</strong> the fundamental<br />
The cos ϕ concerns exclusively the fundamental frequency and therefore differs<br />
from the power factor PF when there are harmonics in the <strong>installation</strong>.<br />
Interpreting the power factor<br />
An initial indication that there are significant amounts <strong>of</strong> harmonics is a measured<br />
power factor PF that is different (lower) than the measured cos ϕ.<br />
5.2 Crest factor<br />
Definition<br />
The crest factor is the ratio between the value <strong>of</strong> the peak current or voltage (Im or<br />
Um) and its rms value.<br />
b For a sinusoidal signal, the crest factor is therefore equal to 2.<br />
b For a non-sinusoidal signal, the crest factor can be either greater than or less<br />
than 2.<br />
In the latter case, the crest factor signals divergent peak values with respect to the<br />
rms value.<br />
Interpretation <strong>of</strong> the crest factor<br />
The typical crest factor for the current drawn by non-linear loads is much higher<br />
than 2. It is generally between 1.5 and 2 and can even reach 5 in critical cases.<br />
A high crest factor signals high transient overcurrents which, when detected by<br />
protection devices, can cause nuisance tripping.<br />
5.3 Power values and harmonics<br />
Active power<br />
The active power P <strong>of</strong> a signal comprising harmonics is the sum <strong>of</strong> the active<br />
powers resulting from the currents and voltages <strong>of</strong> the same order.<br />
Reactive power<br />
Reactive power is defined exclusively in terms <strong>of</strong> the fundamental, i.e.<br />
Q = U1 x I1 x sinϕ1 Distortion power<br />
When harmonics are present, the distortion power D is defined as<br />
D = (S 2 - P 2 - Q 2 ) 1/2 where S is the apparent power.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
M11<br />
© Schneider Electric - all rights reserved
M12<br />
© Schneider Electric - all rights reserved<br />
M - Harmonic management<br />
1<br />
100<br />
33<br />
20<br />
U(t)<br />
H %<br />
0<br />
1<br />
2 3 4 5 6<br />
Fig. M12 : Harmonic spectrum <strong>of</strong> a rectangular signal, for a<br />
voltage U (t)<br />
t<br />
h<br />
5 Essential indicators <strong>of</strong> harmonic<br />
distortion and measurement<br />
principles<br />
5.4 Harmonic spectrum and harmonic distortion<br />
Principle<br />
Each type <strong>of</strong> device causing harmonics draws a particular form <strong>of</strong> harmonic current<br />
(amplitude and phase displacement).<br />
These values, notably the amplitude for each harmonic order, are essential for<br />
analysis.<br />
Individual harmonic distortion (or harmonic distortion <strong>of</strong><br />
order h)<br />
The individual harmonic distortion is defined as the percentage <strong>of</strong> harmonics for<br />
order h with respect to the fundamental.<br />
u % 100 Uh<br />
h(<br />
) =<br />
U1<br />
or<br />
I<br />
i h % 100 h<br />
( ) =<br />
I1<br />
Harmonic spectrum<br />
By representing the amplitude <strong>of</strong> each harmonic order with respect to its frequency, it<br />
is possible to obtain a graph called the harmonic spectrum.<br />
Figure M12 shows an example <strong>of</strong> the harmonic spectrum for a rectangular signal.<br />
Rms value<br />
The rms value <strong>of</strong> the voltage and current can be calculated as a function <strong>of</strong> the rms<br />
value <strong>of</strong> the various harmonic orders.<br />
∞<br />
∑ h<br />
=<br />
2<br />
h 1<br />
Irms = I<br />
and<br />
∞<br />
rms = ∑ h<br />
=<br />
2<br />
U U<br />
h 1<br />
5.5 Total harmonic distortion (THD)<br />
The term THD means Total Harmonic Distortion and is a widely used notion in<br />
defining the level <strong>of</strong> harmonic content in alternating signals.<br />
Definition <strong>of</strong> THD<br />
For a signal y, the THD is defined as:<br />
∞<br />
∑yh<br />
h=<br />
THD =<br />
y<br />
2<br />
2<br />
1<br />
This complies with the definition given in standard IEC 61000-2-2.<br />
Note that the value can exceed 1.<br />
According to the standard, the variable h can be limited to 50. The THD is the means<br />
to express as a single number the distortion affecting a current or voltage flowing at a<br />
given point in the <strong>installation</strong>.<br />
The THD is generally expressed as a percentage.<br />
Current or voltage THD<br />
For current harmonics, the equation is:<br />
∞<br />
∑Ih<br />
h=<br />
THDi =<br />
I<br />
2<br />
2<br />
1<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
M - Harmonic management<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
PF<br />
cos ϕ<br />
0<br />
50<br />
Figure L13 Fig. shows M13 : a Variation graph <strong>of</strong> in<br />
PF<br />
THDu = 0<br />
cosϕ<br />
100<br />
150<br />
THDi<br />
(%)<br />
as a function <strong>of</strong> THDI.<br />
as a function <strong>of</strong> the THDi, where<br />
5 Essential indicators <strong>of</strong> harmonic<br />
distortion and measurement<br />
principles<br />
The equation below is equivalent to the above, but easier and more direct when the<br />
total rms value is available:<br />
THDi = ⎛ ⎞<br />
⎜ ⎟<br />
⎝ ⎠<br />
−<br />
2<br />
Irms<br />
1<br />
I1<br />
For voltage harmonics, the equation is:<br />
THDu<br />
=<br />
∞<br />
∑Uh h=<br />
U<br />
2<br />
2<br />
1<br />
Relation between power factor and THD (see Fig. M13)<br />
When the voltage is sinusoidal or virtually sinusoidal, it may be said that:<br />
P ≈ P1 = U 1. I1.cosϕ1 Consequently : PF = P<br />
S<br />
as:<br />
I<br />
I<br />
1 =<br />
rms<br />
U 1. I1.cosϕ<br />
≈<br />
1<br />
U 1. Irms<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
1<br />
2<br />
1+ THDi<br />
cosϕ<br />
hence: PF ≈ 1<br />
2<br />
1+ THDi<br />
Figure M13 L13 shows a graph <strong>of</strong> PF<br />
as a function <strong>of</strong> THDi. THDI.<br />
cosϕ<br />
5.6 Usefulness <strong>of</strong> the various indicators<br />
The THDu characterises the distortion <strong>of</strong> the voltage wave.<br />
Below are a number <strong>of</strong> THDu values and the corresponding phenomena in the<br />
<strong>installation</strong>:<br />
b THDu under 5% - normal situation, no risk <strong>of</strong> malfunctions<br />
b 5 to 8% - significant harmonic pollution, some malfunctions are possible<br />
b Higher than 8% - major harmonic pollution, malfunctions are probable. In-depth<br />
analysis and the <strong>installation</strong> <strong>of</strong> attenuation devices are required<br />
The THDi characterises the distortion <strong>of</strong> the current wave.<br />
The disturbing device is located by measuring the THDi on the incomer and each<br />
outgoer <strong>of</strong> the various circuits and thus following the harmonic trail.<br />
Below are a number <strong>of</strong> THDi values and the corresponding phenomena in the<br />
<strong>installation</strong>:<br />
b THDi under 10% - normal situation, no risk <strong>of</strong> malfunctions<br />
b 10 to 50% - significant harmonic pollution with a risk <strong>of</strong> temperature rise and the<br />
resulting need to oversize cables and sources<br />
b Higher than 50% - major harmonic pollution, malfunctions are probable. In-depth<br />
analysis and the <strong>installation</strong> <strong>of</strong> attenuation devices are required<br />
Power factor PF<br />
Used to evaluate the necessary oversizing for the power source <strong>of</strong> the <strong>installation</strong>.<br />
Crest factor<br />
Used to characterise the aptitude <strong>of</strong> a generator (or UPS) to supply high<br />
instantaneous currents. For example, computer equipment draws highly distorted<br />
current for which the crest factor can reach 3 to 5.<br />
Spectrum (decomposition <strong>of</strong> the signal into frequencies)<br />
It provides a different representation <strong>of</strong> <strong>electrical</strong> signals and can be used to evaluate<br />
their distortion.<br />
M13<br />
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M - Harmonic management<br />
6 Measuring the indicators<br />
6.1 Devices used to measure the indicators<br />
Device selection<br />
The traditional observation and measurement methods include:<br />
b Observations using an oscilloscope<br />
An initial indication on the distortion affecting a signal can be obtained by viewing the<br />
current or the voltage on an oscilloscope.<br />
The waveform, when it diverges from a sinusoidal, clearly indicates the presence <strong>of</strong><br />
harmonics. Current and voltage peaks can be viewed.<br />
Note, however, that this method does not <strong>of</strong>fer precise quantification <strong>of</strong> the harmonic<br />
components<br />
b Analogue spectral analysers<br />
They are made up <strong>of</strong> passband filters coupled with an rms voltmeter. They <strong>of</strong>fer<br />
mediocre performance and do not provide information on phase displacement.<br />
Only the recent digital analysers can determine sufficiently precisely the<br />
values <strong>of</strong> all the mentioned indicators.<br />
Functions <strong>of</strong> digital analysers<br />
The microprocessors in digital analysers:<br />
b Calculate the values <strong>of</strong> the harmonic indicators (power factor, crest factor,<br />
distortion power, THD)<br />
b Carry out various complementary functions (corrections, statistical detection,<br />
measurement management, display, communication, etc.)<br />
b In multi-channel analysers, supply virtually in real time the simultaneous spectral<br />
decomposition <strong>of</strong> the currents and voltages<br />
Analyser operation and data processing<br />
The analogue signals are converted into a series <strong>of</strong> numerical values.<br />
Using this data, an algorithm implementing the Fast Fourier Transform (FFT)<br />
calculates the amplitudes and the phases <strong>of</strong> the harmonics over a large number <strong>of</strong><br />
time windows.<br />
Most digital analysers measure harmonics up to order 20 or 25 when calculating the<br />
THD.<br />
Processing <strong>of</strong> the successive values calculated using the FFT (smoothing,<br />
classification, statistics) can be carried out by the measurement device or by external<br />
s<strong>of</strong>tware.<br />
6.2 Procedures for harmonic analysis <strong>of</strong> distribution<br />
networks<br />
Measurements are carried out on industrial or commercial site:<br />
b Preventively, to obtain an overall idea on distribution-network status (network map)<br />
b In view <strong>of</strong> corrective action:<br />
v To determine the origin <strong>of</strong> a disturbance and determine the solutions required to<br />
eliminate it<br />
v To check the validity <strong>of</strong> a solution (followed by modifications in the distribution<br />
network to check the reduction in harmonics)<br />
Operating mode<br />
The current and voltage are studied:<br />
b At the supply source<br />
b On the busbars <strong>of</strong> the main distribution switchboard (or on the MV busbars)<br />
b On each outgoing circuit in the main distribution switchboard (or on the<br />
MV busbars)<br />
For the measurements, it is necessary to know the precise operating conditions<br />
<strong>of</strong> the <strong>installation</strong> and particularly the status <strong>of</strong> the capacitor banks (operating, not<br />
operating, the number <strong>of</strong> disconnected steps).<br />
Analysis results<br />
b Determine any necessary derating <strong>of</strong> equipment in the <strong>installation</strong> or<br />
b Quantify any necessary harmonic protection and filtering systems to be installed in<br />
the distribution network<br />
b Enable comparison between the measured values and the reference values <strong>of</strong> the<br />
utility (maximum harmonic values, acceptable values, reference values)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
M - Harmonic management<br />
6 Measuring the indicators<br />
Use <strong>of</strong> measurement devices<br />
Measurement devices serve to show both the instantaneous and long-term effects <strong>of</strong><br />
harmonics. Analysis requires values spanning durations ranging from a few seconds<br />
to several minutes over observation periods <strong>of</strong> a number <strong>of</strong> days.<br />
The required values include:<br />
b The amplitudes <strong>of</strong> the harmonic currents and voltages<br />
b The individual harmonic content <strong>of</strong> each harmonic order <strong>of</strong> the current and voltage<br />
b The THD for the current and voltage<br />
b Where applicable, the phase displacement between the harmonic voltage and<br />
current <strong>of</strong> the same harmonic order and the phase <strong>of</strong> the harmonics with respect to a<br />
common reference (e.g. the fundamental voltage)<br />
6.3 Keeping a close eye on harmonics<br />
The harmonic indicators can be measured:<br />
b Either by devices permanently installed in the distribution network<br />
b Or by an expert present at least a half day on the site (limited perception)<br />
Permanent devices are preferable<br />
For a number <strong>of</strong> reasons, the <strong>installation</strong> <strong>of</strong> permanent measurement devices in the<br />
distribution network is preferable.<br />
b The presence <strong>of</strong> an expert is limited in time. Only a number <strong>of</strong> measurements at<br />
different points in the <strong>installation</strong> and over a sufficiently long period (one week to a<br />
month) provide an overall view <strong>of</strong> operation and take into account all the situations<br />
that can occur following:<br />
v Fluctuations in the supply source<br />
v Variations in the operation <strong>of</strong> the <strong>installation</strong><br />
v The addition <strong>of</strong> new equipment in the <strong>installation</strong><br />
b Measurement devices installed in the distribution network prepare and facilitate the<br />
diagnosis <strong>of</strong> the experts, thus reducing the number and duration <strong>of</strong> their visits<br />
b Permanent measurement devices detect any new disturbances arising following<br />
the <strong>installation</strong> <strong>of</strong> new equipment, the implementation <strong>of</strong> new operating modes or<br />
fluctuations in the supply network<br />
Take advantage <strong>of</strong> built-in measurement and detection devices<br />
Measurement and detection devices built into the <strong>electrical</strong> distribution equipment:<br />
b For an overall evaluation <strong>of</strong> network status (preventive analysis), avoid:<br />
v Renting measurement equipment<br />
v Calling in experts<br />
v Having to connect and disconnect the measurement equipment.<br />
For the overall evaluation <strong>of</strong> network status, the analysis on the main low-voltage<br />
distribution switchboards (MLVS) can <strong>of</strong>ten be carried out by the incoming device<br />
and/or the measurement devices equipping each outgoing circuit<br />
b For corrective action, are the means to:<br />
v Determine the operating conditions at the time <strong>of</strong> the incident<br />
v Draw up a map <strong>of</strong> the distribution network and evaluate the implemented solution<br />
The diagnosis is improved by the use <strong>of</strong> equipment intended for the studied problem.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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M16<br />
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M - Harmonic management<br />
PowerLogic System with Power Meter and<br />
Circuit Monitor, Micrologic <strong>of</strong>fer a complete<br />
range <strong>of</strong> devices for the detection <strong>of</strong> harmonic<br />
distortion<br />
Fig. M14 : Circuit monitor<br />
Fig. M15 : Micrologic H control unit with harmonic metering for<br />
Masterpact NT and NW circuit-breakers<br />
7 Detection devices<br />
Measurements are the first step in gaining control over harmonic pollution.<br />
Depending on the conditions in each <strong>installation</strong>, different types <strong>of</strong> equipment<br />
provide the necessary solution.<br />
Power-monitoring units<br />
Power Meter and Circuit Monitor in the PowerLogic System<br />
These products <strong>of</strong>fer high-performance measurement capabilities for low and<br />
medium-voltage distribution networks. They are digital units that include powerquality<br />
monitoring functions.<br />
PowerLogic System is a complete <strong>of</strong>fer comprising Power Meter (PM) and Circuit<br />
Monitor (CM). This highly modular <strong>of</strong>fer covers needs ranging from the most simple<br />
(Power Meter) up to highly complex requirements (Circuit Monitor). These products<br />
can be used in new or existing <strong>installation</strong>s where the level <strong>of</strong> power quality must be<br />
excellent. The operating mode can be local and/or remote.<br />
Depending on its position in the distribution network, a Power Meter provides an initial<br />
indication on power quality. The main measurements carried out by a Power Meter are:<br />
b Current and voltage THD<br />
b Power factor<br />
Depending on the version, these measurements can be combined with timestamping<br />
and alarm functions.<br />
A Circuit Monitor (see Fig. M14) carries out a detailed analysis <strong>of</strong> power quality<br />
and also analyses disturbances on the distribution network. The main functions <strong>of</strong> a<br />
Circuit Monitor are:<br />
b Measurement <strong>of</strong> over 100 <strong>electrical</strong> parameters<br />
b Storage in memory and time-stamping <strong>of</strong> minimum and maximum values for each<br />
<strong>electrical</strong> parameter<br />
b Alarm functions tripped by <strong>electrical</strong> parameter values<br />
b Recording <strong>of</strong> event data<br />
b Recording <strong>of</strong> current and voltage disturbances<br />
b Harmonic analysis<br />
b Waveform capture (disturbance monitoring)<br />
Micrologic - a power-monitoring unit built into the circuit-breaker<br />
For new <strong>installation</strong>s, the Micrologic H control unit (see Fig. M15), an integral part<br />
<strong>of</strong> Masterpact power circuit-breakers, is particularly useful for measurements at the<br />
head <strong>of</strong> an <strong>installation</strong> or on large outgoing circuits.<br />
The Micrologic H control unit <strong>of</strong>fers precise analysis <strong>of</strong> power quality and detailed<br />
diagnostics on events. It is <strong>design</strong>ed for operation in conjunction with a switchboard<br />
display unit or a supervisor. It can:<br />
b Measure current, voltage, active and reactive power<br />
b Measure current and voltage THD<br />
b Display the amplitude and phase <strong>of</strong> current and voltage harmonics up to the 51 st order<br />
b Carry out waveform capture (disturbance monitoring)<br />
The functions <strong>of</strong>fered by the Micrologic H control unit are equivalent to those <strong>of</strong> a<br />
Circuit Monitor.<br />
Operation <strong>of</strong> power-monitoring units<br />
S<strong>of</strong>tware for remote operation and analysis<br />
In the more general framework <strong>of</strong> a distribution network requiring monitoring,<br />
the possibility <strong>of</strong> interconnecting these various devices can be <strong>of</strong>fered in a<br />
communication network, thus making it possible to centralise information and obtain<br />
an overall view <strong>of</strong> disturbances throughout the distribution network.<br />
Depending on the application, the operator can then carry out measurements in real<br />
time, calculate demand values, run waveform captures, anticipate on alarms, etc.<br />
The power-monitoring units transmit all the available data over either a Modbus,<br />
Digipact or Ethernet network.<br />
The essential goal <strong>of</strong> this system is to assist in identifying and planning maintenance<br />
work. It is an effective means to reduce servicing time and the cost <strong>of</strong> temporarily<br />
installing devices for on-site measurements or the sizing <strong>of</strong> equipment (filters).<br />
Supervision s<strong>of</strong>tware SMS<br />
SMS is a very complete s<strong>of</strong>tware used to analyse distribution networks, in conjunction<br />
with the products in the PowerLogic System. Installed on a standard PC, it can:<br />
b Display measurements in real time<br />
b Display historical logs over a given period<br />
b Select the manner in which data is presented (tables, various curves)<br />
b Carry out statistical processing <strong>of</strong> data (display bar charts)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
M - Harmonic management<br />
8 Solutions to attenuate<br />
harmonics<br />
There are three different types <strong>of</strong> solutions to attenuate harmonics:<br />
b Modifications in the <strong>installation</strong><br />
b Special devices in the supply system<br />
b Filtering<br />
8.1 Basic solutions<br />
To limit the propagation <strong>of</strong> harmonics in the distribution network, different solutions<br />
are available and should be taken into account particularly when <strong>design</strong>ing a new<br />
<strong>installation</strong>.<br />
Position the non-linear loads upstream in the system<br />
Overall harmonic disturbances increase as the short-circuit power decreases.<br />
All economic considerations aside, it is preferable to connect the non-linear loads as<br />
far upstream as possible (see Fig. M16).<br />
Fig. M16 : Non-linear loads positioned as far upstream as possible (recommended layout)<br />
Group the non-linear loads<br />
When preparing the single-line diagram, the non-linear devices should be separated<br />
from the others (see Fig. M17). The two groups <strong>of</strong> devices should be supplied by<br />
different sets <strong>of</strong> busbars.<br />
Yes<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Z 1<br />
Non-linear<br />
loads<br />
Line impedances<br />
No<br />
Sensitive<br />
loads<br />
Non-linear<br />
load 1<br />
Non-linear<br />
load 2<br />
Fig. M17 : Grouping <strong>of</strong> non-linear loads and connection as far upstream as possible<br />
(recommended layout)<br />
Create separate sources<br />
In attempting to limit harmonics, an additional improvement can be obtained by<br />
creating a source via a separate transformer as indicated in the Figure M18 next<br />
page.<br />
The disadvantage is the increase in the cost <strong>of</strong> the <strong>installation</strong>.<br />
Z 2<br />
Where impedance<br />
Z 1 < Z 2<br />
Sensitive<br />
loads<br />
M17<br />
© Schneider Electric - all rights reserved
M18<br />
© Schneider Electric - all rights reserved<br />
M - Harmonic management<br />
8 Solutions to attenuate<br />
harmonics<br />
MV<br />
network<br />
Fig. M18 : Supply <strong>of</strong> non-linear loads via a separate transformer<br />
Transformers with special connections<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Non-linear<br />
loads<br />
Linear<br />
loads<br />
Different transformer connections can eliminate certain harmonic orders, as<br />
indicated in the examples below:<br />
b A Dyd connection suppresses 5 th and 7 th harmonics (see Fig. M19)<br />
b A Dy connection suppresses the 3 rd harmonic<br />
b A DZ 5 connection suppresses the 5 th harmonic<br />
h11, h13<br />
h5, h7, h11, h13<br />
h5, h7, h11, h13<br />
Fig. M19 : A Dyd transformer blocks propagation <strong>of</strong> the 5 th and 7 th harmonics to the upstream<br />
network<br />
Install reactors<br />
When variable-speed drives are supplied, it is possible to smooth the current<br />
by installing line reactors. By increasing the impedance <strong>of</strong> the supply circuit, the<br />
harmonic current is limited.<br />
Installation <strong>of</strong> harmonic suppression reactors on capacitor banks increases the<br />
impedance <strong>of</strong> the reactor/capacitor combination for high-order harmonics.<br />
This avoids resonance and protects the capacitors.<br />
Select the suitable system earthing arrangement<br />
TNC system<br />
In the TNC system, a single conductor (PEN) provides protection in the event <strong>of</strong> an<br />
earth fault and the flow <strong>of</strong> unbalance currents.<br />
Under steady-state conditions, the harmonic currents flow in the PEN. The latter,<br />
however, has a certain impedance with as a result slight differences in potential (a<br />
few volts) between devices that can cause electronic equipment to malfunction.<br />
The TNC system must therefore be reserved for the supply <strong>of</strong> power circuits at the<br />
head <strong>of</strong> the <strong>installation</strong> and must not be used to supply sensitive loads.<br />
TNS system<br />
This system is recommended if harmonics are present.<br />
The neutral conductor and the protection conductor PE are completely separate and<br />
the potential throughout the distribution network is therefore more uniform.<br />
8.2 Harmonic filtering<br />
In cases where the preventive action presented above is insufficient, it is necessary<br />
to equip the <strong>installation</strong> with filtering systems.<br />
There are three types <strong>of</strong> filters:<br />
b Passive<br />
b Active<br />
b Hybrid
M - Harmonic management<br />
I har<br />
Non-linear<br />
load<br />
Fig. M20 : Operating principle <strong>of</strong> a passive filter<br />
I har<br />
Non-linear<br />
load<br />
Filter<br />
AHC<br />
Fig. M22 : Operating principle <strong>of</strong> a hybrid filter<br />
Is<br />
Iact<br />
Fig. M21 : Operating principle <strong>of</strong> an active filter<br />
I har<br />
Non-linear<br />
load<br />
AHC<br />
Iact<br />
Hybride filter<br />
Linear<br />
load<br />
Is<br />
Linear<br />
load<br />
8 Solutions to attenuate<br />
harmonics<br />
Passive filters<br />
Typical applications<br />
b Industrial <strong>installation</strong>s with a set <strong>of</strong> non-linear loads representing more than<br />
200 kVA (variable-speed drives, UPSs, rectifiers, etc.)<br />
b Installations requiring power-factor correction<br />
b Installations where voltage distortion must be reduced to avoid disturbing sensitive<br />
loads<br />
b Installations where current distortion must be reduced to avoid overloads<br />
Operating principle<br />
An LC circuit, tuned to each harmonic order to be filtered, is installed in parallel with<br />
the non-linear load (see Fig. M20). This bypass circuit absorbs the harmonics, thus<br />
avoiding their flow in the distribution network.<br />
<strong>General</strong>ly speaking, the passive filter is tuned to a harmonic order close to the order<br />
to be eliminated. Several parallel-connected branches <strong>of</strong> filters can be used if a<br />
significant reduction in the distortion <strong>of</strong> a number <strong>of</strong> harmonic orders is required.<br />
Active filters (active harmonic conditioner)<br />
Typical applications<br />
b Commercial <strong>installation</strong>s with a set <strong>of</strong> non-linear loads representing less than<br />
200 kVA (variable-speed drives, UPSs, <strong>of</strong>fice equipment, etc.)<br />
b Installations where current distortion must be reduced to avoid overloads.<br />
Operating principle<br />
These systems, comprising power electronics and installed in series or parallel with<br />
the non-linear load, compensate the harmonic current or voltage drawn by the load.<br />
Figure M21 shows a parallel-connected active harmonic conditioner (AHC)<br />
compensating the harmonic current (Ihar = -Iact).<br />
The AHC injects in opposite phase the harmonics drawn by the non-linear load, such<br />
that the line current Is remains sinusoidal.<br />
Hybrid filters<br />
Typical applications<br />
b Industrial <strong>installation</strong>s with a set <strong>of</strong> non-linear loads representing more than<br />
200 kVA (variable-speed drives, UPSs, rectifiers, etc.)<br />
b Installations requiring power-factor correction<br />
b Installations where voltage distortion must be reduced to avoid disturbing sensitive<br />
loads<br />
b Installations where current distortion must be reduced to avoid overloads<br />
b Installations where strict limits on harmonic emissions must be met<br />
Operating principle<br />
Passive and active filters are combined in a single system to constitute a hybrid filter<br />
(see Fig. M22). This new filtering solution <strong>of</strong>fers the advantages <strong>of</strong> both types <strong>of</strong><br />
filters and covers a wide range <strong>of</strong> power and performance levels.<br />
Selection criteria<br />
Passive filter<br />
It <strong>of</strong>fers both power-factor correction and high current-filtering capacity.<br />
Passive filters also reduce the harmonic voltages in <strong>installation</strong>s where the supply<br />
voltage is disturbed. If the level <strong>of</strong> reactive power supplied is high, it is advised to turn<br />
<strong>of</strong>f the passive filter at times when the percent load is low.<br />
Preliminary studies for a filter must take into account the possible presence <strong>of</strong> a<br />
power factor correction capacitor bank which may have to be eliminated.<br />
Active harmonic conditioners<br />
They filter harmonics over a wide range <strong>of</strong> frequencies and can adapt to any type <strong>of</strong><br />
load.<br />
On the other hand, power ratings are low.<br />
Hybrid filters<br />
They combine the performance <strong>of</strong> both active and passive filters.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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M - Harmonic management<br />
A complete set <strong>of</strong> services can be <strong>of</strong>fered to<br />
eliminate harmonics:<br />
b Installation analysis<br />
b Measurement and monitoring systems<br />
b Filtering solutions<br />
8 Solutions to attenuate<br />
harmonics<br />
8.3 The method<br />
The best solution, in both technical and financial terms, is based on the results <strong>of</strong> an<br />
in-depth study.<br />
Harmonic audit <strong>of</strong> MV and LV networks<br />
By calling on an expert, you are guaranteed that the proposed solution will produce<br />
effective results (e.g. a guaranteed maximum THDu).<br />
A harmonic audit is carried out by an engineer specialised in the disturbances<br />
affecting <strong>electrical</strong> distribution networks and equipped with powerful analysis and<br />
simulation equipment and s<strong>of</strong>tware.<br />
The steps in an audit are the following:<br />
b Measurement <strong>of</strong> disturbances affecting current and phase-to-phase and phaseto-neutral<br />
voltages at the supply source, the disturbed outgoing circuits and the<br />
non-linear loads<br />
b Computer modelling <strong>of</strong> the phenomena to obtain a precise explanation <strong>of</strong> the<br />
causes and determine the best solutions<br />
b A complete audit report presenting:<br />
v The current levels <strong>of</strong> disturbances<br />
v The maximum permissible levels <strong>of</strong> disturbances (IEC 61000, IEC 34, etc.)<br />
b A proposal containing solutions with guaranteed levels <strong>of</strong> performance<br />
b Finally, implementation <strong>of</strong> the selected solution, using the necessary means and<br />
resources.<br />
The entire audit process is certified ISO 9002.<br />
8.4 Specific products<br />
Passive filters<br />
Passive filters are made up <strong>of</strong> coils and capacitors set up in resonant circuits tuned<br />
to the specific harmonic order that must be eliminated.<br />
A system may comprise a number <strong>of</strong> filters to eliminate several harmonic orders.<br />
Suitable for 400 V three-phase voltages, the power ratings can reach:<br />
b 265 kvar / 470 A for harmonic order 5<br />
b 145 kvar / 225 A for harmonic order 7<br />
b 105 kvar / 145 A for harmonic order 11<br />
Passive filters can be created for all voltage and current levels.<br />
Active filters<br />
b SineWave active harmonic conditioners<br />
v Suitable for 400 V three-phase voltages, they can deliver between 20 and 120 A<br />
per phase<br />
v SineWave covers all harmonic orders from 2 to 25. Conditioning can be total or<br />
target specific harmonic orders<br />
v Attenuation: THDi load / THDi upstream greater than 10 at rated capacity<br />
v Functions include power factor correction, conditioning <strong>of</strong> zero-sequence<br />
harmonics, diagnostics and maintenance system, parallel connection, remote<br />
control, Ibus/RS485 communication interface<br />
b Accusine active filters<br />
v Suitable for 400 and 480 V three-phase voltages, they can filter between 50 and 30<br />
A per phase<br />
v All harmonic orders up to 50 are filtered<br />
v Functions include power factor correction, parallel connection, instantaneous<br />
response to load variations<br />
Hybrid filters<br />
These filters combine the advantages <strong>of</strong> both a passive filter and the SineWave<br />
active harmonic conditioner in a single system.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
a -<br />
b -<br />
Fig. N35 : Compact fluorescent lamps [a] standard,<br />
[b] induction<br />
4 Lighting circuits<br />
A source <strong>of</strong> comfort and productivity, lighting represents 15% <strong>of</strong> the quantity <strong>of</strong><br />
electricity consumed in industry and 40% in buildings. The quality <strong>of</strong> lighting (light<br />
stability and continuity <strong>of</strong> service) depends on the quality <strong>of</strong> the <strong>electrical</strong> energy<br />
thus consumed. The supply <strong>of</strong> <strong>electrical</strong> power to lighting networks has therefore<br />
assumed great importance.<br />
To help with their <strong>design</strong> and simplify the selection <strong>of</strong> appropriate protection devices,<br />
an analysis <strong>of</strong> the different lamp technologies is presented. The distinctive features<br />
<strong>of</strong> lighting circuits and their impact on control and protection devices are discussed.<br />
Recommendations relative to the difficulties <strong>of</strong> lighting circuit implementation are given.<br />
4.1 The different lamp technologies<br />
Artificial luminous radiation can be produced from <strong>electrical</strong> energy according to two<br />
principles: incandescence and electroluminescence.<br />
Incandescence is the production <strong>of</strong> light via temperature elevation. The most<br />
common example is a filament heated to white state by the circulation <strong>of</strong> an <strong>electrical</strong><br />
current. The energy supplied is transformed into heat by the Joule effect and into<br />
luminous flux.<br />
Luminescence is the phenomenon <strong>of</strong> emission by a material <strong>of</strong> visible or almost<br />
visible luminous radiation. A gas (or vapors) subjected to an <strong>electrical</strong> discharge<br />
emits luminous radiation (Electroluminescence <strong>of</strong> gases).<br />
Since this gas does not conduct at normal temperature and pressure, the discharge<br />
is produced by generating charged particles which permit ionization <strong>of</strong> the gas. The<br />
nature, pressure and temperature <strong>of</strong> the gas determine the light spectrum.<br />
Photoluminescence is the luminescence <strong>of</strong> a material exposed to visible or almost<br />
visible radiation (ultraviolet, infrared).<br />
When the substance absorbs ultraviolet radiation and emits visible radiation which<br />
stops a short time after energization, this is fluorescence.<br />
Incandescent lamps<br />
Incandescent lamps are historically the oldest and the most <strong>of</strong>ten found in common<br />
use.<br />
They are based on the principle <strong>of</strong> a filament rendered incandescent in a vacuum or<br />
neutral atmosphere which prevents combustion.<br />
A distinction is made between:<br />
b Standard bulbs<br />
These contain a tungsten filament and are filled with an inert gas (nitrogen and<br />
argon or krypton).<br />
b Halogen bulbs<br />
These also contain a tungsten filament, but are filled with a halogen compound<br />
and an inert gas (krypton or xenon). This halogen compound is responsible for the<br />
phenomenon <strong>of</strong> filament regeneration, which increases the service life <strong>of</strong> the lamps<br />
and avoids them blackening. It also enables a higher filament temperature and<br />
therefore greater luminosity in smaller-size bulbs.<br />
The main disadvantage <strong>of</strong> incandescent lamps is their significant heat dissipation,<br />
resulting in poor luminous efficiency.<br />
Fluorescent lamps<br />
This family covers fluorescent tubes and compact fluorescent lamps. Their<br />
technology is usually known as “low-pressure mercury”.<br />
In fluorescent tubes, an <strong>electrical</strong> discharge causes electrons to collide with<br />
ions <strong>of</strong> mercury vapor, resulting in ultraviolet radiation due to energization <strong>of</strong> the<br />
mercury atoms. The fluorescent material, which covers the inside <strong>of</strong> the tubes, then<br />
transforms this radiation into visible light.<br />
Fluorescent tubes dissipate less heat and have a longer service life than<br />
incandescent lamps, but they do need an ignition device called a “starter” and a<br />
device to limit the current in the arc after ignition. This device called “ballast” is<br />
usually a choke placed in series with the arc.<br />
Compact fluorescent lamps are based on the same principle as a fluorescent tube.<br />
The starter and ballast functions are provided by an electronic circuit (integrated in<br />
the lamp) which enables the use <strong>of</strong> smaller tubes folded back on themselves.<br />
Compact fluorescent lamps (see Fig. N35) were developed to replace incandescent<br />
lamps: They <strong>of</strong>fer significant energy savings (15 W against 75 W for the same level <strong>of</strong><br />
brightness) and an increased service life.<br />
Lamps known as “induction” type or “without electrodes” operate on the principle <strong>of</strong><br />
ionization <strong>of</strong> the gas present in the tube by a very high frequency electromagnetic<br />
field (up to 1 GHz). Their service life can be as long as 100,000 hrs.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
N27<br />
© Schneider Electric - all rights reserved
N28<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
Fig. N36 : Discharge lamps<br />
Discharge lamps (see Fig. N36)<br />
The light is produced by an <strong>electrical</strong> discharge created between two electrodes within<br />
a gas in a quartz bulb. All these lamps therefore require a ballast to limit the current in<br />
the arc. A number <strong>of</strong> technologies have been developed for different applications.<br />
Low-pressure sodium vapor lamps have the best light output, however the color<br />
rendering is very poor since they only have a monochromatic orange radiation.<br />
High-pressure sodium vapor lamps produce a white light with an orange tinge.<br />
In high-pressure mercury vapor lamps, the discharge is produced in a quartz or<br />
ceramic bulb at high pressure. These lamps are called “fluorescent mercury discharge<br />
lamps”. They produce a characteristically bluish white light.<br />
Metal halide lamps are the latest technology. They produce a color with a broad color<br />
spectrum. The use <strong>of</strong> a ceramic tube <strong>of</strong>fers better luminous efficiency and better color<br />
stability.<br />
Light Emitting Diodes (LED)<br />
The principle <strong>of</strong> light emitting diodes is the emission <strong>of</strong> light by a semi-conductor<br />
as an <strong>electrical</strong> current passes through it. LEDs are commonly found in numerous<br />
applications, but the recent development <strong>of</strong> white or blue diodes with a high light<br />
output opens new perspectives, especially for signaling (traffic lights, exit signs or<br />
emergency lighting).<br />
LEDs are low-voltage and low-current devices, thus suitable for battery-supply.<br />
A converter is required for a line power supply.<br />
The advantage <strong>of</strong> LEDs is their low energy consumption. As a result, they operate<br />
at a very low temperature, giving them a very long service life. Conversely, a simple<br />
diode has a weak light intensity. A high-power lighting <strong>installation</strong> therefore requires<br />
connection <strong>of</strong> a large number <strong>of</strong> units in series and parallel.<br />
Technology Application Advantages Disadvantages<br />
Standard - Domestic use - Direct connection without - Low luminous efficiency and<br />
incandescent - Localized decorative intermediate switchgear high electricity consumption<br />
lighting - Reasonable purchase price - Significant heat dissipation<br />
- Compact size - Short service life<br />
- Instantaneous lighting<br />
- Good color rendering<br />
Halogen - Spot lighting - Direct connection - Average luminous efficiency<br />
incandescent - Intense lighting - Instantaneous efficiency<br />
- Excellent color rendering<br />
Fluorescent tube - Shops, <strong>of</strong>fices, workshops - High luminous efficiency - Low light intensity <strong>of</strong> single unit<br />
- Outdoors - Average color rendering - Sensitive to extreme temperatures<br />
Compact - Domestic use - Good luminous efficiency - High initial investment<br />
fluorescent lamp - Offices - Good color rendering compared to incandescent lamps<br />
- Replacement <strong>of</strong><br />
incandescent lamps<br />
HP mercury vapor - Workshops, halls, hangars - Good luminous efficiency - Lighting and relighting time<br />
- Factory floors - Acceptable color rendering <strong>of</strong> a few minutes<br />
- Compact size<br />
- Long service life<br />
High-pressure - Outdoors - Very good luminous efficiency - Lighting and relighting time<br />
sodium - Large halls <strong>of</strong> a few minutes<br />
Low-pressure - Outdoors - Good visibility in foggy weather - Long lighting time (5 min.)<br />
sodium - Emergency lighting - Economical to use - Mediocre color rendering<br />
Metal halide - Large areas - Good luminous efficiency - Lighting and relighting time<br />
- Halls with high ceilings - Good color rendering <strong>of</strong> a few minutes<br />
- Long service life<br />
LED - Signaling (3-color traffic - Insensitive to the number <strong>of</strong> - Limited number <strong>of</strong> colors<br />
lights, “exit” signs and switching operations - Low brightness <strong>of</strong> single unit<br />
emergency lighting) - Low energy consumption<br />
- Low temperature<br />
Technology Power (watt) Efficiency (lumen/watt) Service life (hours)<br />
Standard incandescent 3 – 1,000 10 – 15 1,000 – 2,000<br />
Halogen incandescent 5 – 500 15 – 25 2,000 – 4,000<br />
Fluorescent tube 4 – 56 50 – 100 7,500 – 24,000<br />
Compact fluorescent lamp 5 – 40 50 – 80 10,000 – 20,000<br />
HP mercury vapor 40 – 1,000 25 – 55 16,000 – 24,000<br />
High-pressure sodium 35 – 1,000 40 – 140 16,000 – 24,000<br />
Low-pressure sodium 35 – 180 100 – 185 14,000 – 18,000<br />
Metal halide 30 – 2,000 50 – 115 6,000 – 20,000<br />
LED 0.05 – 0.1 10 – 30 40,000 – 100,000<br />
Fig. N37 : Usage and technical characteristics <strong>of</strong> lighting devices<br />
4 Lighting circuits<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
a]<br />
300<br />
200<br />
100<br />
0<br />
-100<br />
-200<br />
-300<br />
0<br />
b]<br />
300<br />
200<br />
100<br />
0<br />
-100<br />
-200<br />
0.01 0.02<br />
-300<br />
0 0.01 0.02<br />
t (s)<br />
t (s)<br />
Fig. N38 : Shape <strong>of</strong> the voltage supplied by a light dimmer at<br />
50% <strong>of</strong> maximum voltage with the following techniques:<br />
a] “cut-on control”<br />
b] “cut-<strong>of</strong>f control”<br />
4 Lighting circuits<br />
4.2 Electrical characteristics <strong>of</strong> lamps<br />
Incandescent lamps with direct power supply<br />
Due to the very high temperature <strong>of</strong> the filament during operation (up to 2,500 °C),<br />
its resistance varies greatly depending on whether the lamp is on or <strong>of</strong>f. As the cold<br />
resistance is low, a current peak occurs on ignition that can reach 10 to 15 times the<br />
nominal current for a few milliseconds or even several milliseconds.<br />
This constraint affects both ordinary lamps and halogen lamps: it imposes a<br />
reduction in the maximum number <strong>of</strong> lamps that can be powered by devices such as<br />
remote-control switches, modular contactors and relays for busbar trunking.<br />
Extra Low Voltage (ELV) halogen lamps<br />
b Some low-power halogen lamps are supplied with ELV 12 or 24 V, via a<br />
transformer or an electronic converter. With a transformer, the magnetization<br />
phenomenon combines with the filament resistance variation phenomenon at<br />
switch-on. The inrush current can reach 50 to 75 times the nominal current for a few<br />
milliseconds. The use <strong>of</strong> dimmer switches placed upstream significantly reduces this<br />
constraint.<br />
b Electronic converters, with the same power rating, are more expensive than<br />
solutions with a transformer. This commercial handicap is compensated by a greater<br />
ease <strong>of</strong> <strong>installation</strong> since their low heat dissipation means they can be fixed on a<br />
flammable support. Moreover, they usually have built-in thermal protection.<br />
New ELV halogen lamps are now available with a transformer integrated in their<br />
base. They can be supplied directly from the LV line supply and can replace normal<br />
lamps without any special adaptation.<br />
Dimming for incandescent lamps<br />
This can be obtained by varying the voltage applied to the lampere<br />
This voltage variation is usually performed by a device such as a Triac dimmer<br />
switch, by varying its firing angle in the line voltage period. The wave form <strong>of</strong> the<br />
voltage applied to the lamp is illustrated in Figure N38a. This technique known<br />
as “cut-on control” is suitable for supplying power to resistive or inductive circuits.<br />
Another technique suitable for supplying power to capacitive circuits has been<br />
developed with MOS or IGBT electronic components. This techniques varies the<br />
voltage by blocking the current before the end <strong>of</strong> the half-period (see Fig. N38b) and<br />
is known as “cut-<strong>of</strong>f control”.<br />
Switching on the lamp gradually can also reduce, or even eliminate, the current peak<br />
on ignition.<br />
As the lamp current is distorted by the electronic switching, harmonic currents<br />
are produced. The 3<br />
N29<br />
rd harmonic order is predominant, and the percentage <strong>of</strong> 3rd harmonic current related to the maximum fundamental current (at maximum power)<br />
is represented on Figure N39.<br />
Note that in practice, the power applied to the lamp by a dimmer switch can only vary<br />
in the range between 15 and 85% <strong>of</strong> the maximum power <strong>of</strong> the lampere<br />
50.0<br />
45.0<br />
40.0<br />
35.0<br />
30.0<br />
25.0<br />
20.0<br />
15.0<br />
10.0<br />
5.0<br />
0<br />
i3 (%)<br />
0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0<br />
Fig. N39 : Percentage <strong>of</strong> 3 rd harmonic current as a function <strong>of</strong> the power applied to an<br />
incandescent lamp using an electronic dimmer switch<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Power (%)<br />
© Schneider Electric - all rights reserved
N30<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
Fig. N40 : Magnetic ballasts<br />
4 Lighting circuits<br />
According to IEC standard 61000-3-2 setting harmonic emission limits for electric or<br />
electronic systems with current y 16 A, the following arrangements apply:<br />
b Independent dimmers for incandescent lamps with a rated power less than or<br />
equal to 1 kW have no limits applied<br />
b Otherwise, or for incandescent lighting equipment with built-in dimmer or dimmer<br />
built in an enclosure, the maximum permissible 3 rd harmonic current is equal to<br />
2.30 A<br />
Fluorescent lamps with magnetic ballast<br />
Fluorescent tubes and discharge lamps require the intensity <strong>of</strong> the arc to be limited,<br />
and this function is fulfilled by a choke (or magnetic ballast) placed in series with the<br />
bulb itself (see Fig. N40).<br />
This arrangement is most commonly used in domestic applications with a limited<br />
number <strong>of</strong> tubes. No particular constraint applies to the switches.<br />
Dimmer switches are not compatible with magnetic ballasts: the cancellation <strong>of</strong> the<br />
voltage for a fraction <strong>of</strong> the period interrupts the discharge and totally extinguishes<br />
the lampere<br />
The starter has a dual function: preheating the tube electrodes, and then generating<br />
an overvoltage to ignite the tube. This overvoltage is generated by the opening <strong>of</strong> a<br />
contact (controlled by a thermal switch) which interrupts the current circulating in the<br />
magnetic ballast.<br />
During operation <strong>of</strong> the starter (approx. 1 s), the current drawn by the luminaire is<br />
approximately twice the nominal current.<br />
Since the current drawn by the tube and ballast assembly is essentially inductive, the<br />
power factor is very low (on average between 0.4 and 0.5). In <strong>installation</strong>s consisting<br />
<strong>of</strong> a large number <strong>of</strong> tubes, it is necessary to provide compensation to improve the<br />
power factor.<br />
For large lighting <strong>installation</strong>s, centralized compensation with capacitor banks is a<br />
possible solution, but more <strong>of</strong>ten this compensation is included at the level <strong>of</strong> each<br />
luminaire in a variety <strong>of</strong> different layouts (see Fig. N41).<br />
a] Ballast b] C Ballast c]<br />
a<br />
C Lamp a<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Lamp<br />
a<br />
C<br />
Ballast<br />
Lamp<br />
Ballast Lamp<br />
Compensation layout Application Comments<br />
Without compensation Domestic Single connection<br />
Parallel [a] Offices, workshops,<br />
superstores<br />
Risk <strong>of</strong> overcurrents for control devices<br />
Series [b] Choose capacitors with high<br />
operating voltage (450 to 480 V)<br />
Duo [c] Avoids flicker<br />
Fig. N41 : The various compensation layouts: a] parallel; b] series; c] dual series also called<br />
“duo” and their fields <strong>of</strong> application<br />
The compensation capacitors are therefore sized so that the global power factor is<br />
greater than 0.85. In the most common case <strong>of</strong> parallel compensation, its capacity<br />
is on average 1 µF for 10 W <strong>of</strong> active power, for any type <strong>of</strong> lampere However, this<br />
compensation is incompatible with dimmer switches.<br />
Constraints affecting compensation<br />
The layout for parallel compensation creates constraints on ignition <strong>of</strong> the lampere<br />
Since the capacitor is initially discharged, switch-on produces an overcurrent.<br />
An overvoltage also appears, due to the oscillations in the circuit made up <strong>of</strong> the<br />
capacitor and the power supply inductance.<br />
The following example can be used to determine the orders <strong>of</strong> magnitude.
N - Characteristics <strong>of</strong> particular sources and loads<br />
4 Lighting circuits<br />
Assuming an assembly <strong>of</strong> 50 fluorescent tubes <strong>of</strong> 36 W each:<br />
b Total active power: 1,800 W<br />
b Apparent power: 2 kVA<br />
b Total rms current: 9 A<br />
b Peak current: 13 A<br />
With:<br />
b A total capacity: C = 175 µF<br />
b A line inductance (corresponding to a short-circuit current <strong>of</strong> 5 kA): L = 150 µH<br />
The maximum peak current at switch-on equals:<br />
-6<br />
C<br />
x 10<br />
Ic = Vmax<br />
= 230 2 = A<br />
L<br />
-6<br />
x 10<br />
175<br />
350<br />
150<br />
The theoretical peak current at switch-on can therefore reach 27 times the peak<br />
current during normal operation.<br />
The shape <strong>of</strong> the voltage and current at ignition is given in Figure N42 for switch<br />
closing at the line supply voltage peak.<br />
There is therefore a risk <strong>of</strong> contact welding in electromechanical control devices<br />
(remote-control switch, contactor, circuit-breaker) or destruction <strong>of</strong> solid state<br />
switches with semi-conductors.<br />
600<br />
400<br />
200<br />
0<br />
-200<br />
-400<br />
-600<br />
0<br />
300<br />
200<br />
100<br />
0<br />
-100<br />
-200<br />
(V)<br />
(A)<br />
-300<br />
0<br />
Fig. N42 : Power supply voltage at switch-on and inrush current<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
0.02 0.04 0.06<br />
0.02 0.04 0.06<br />
In reality, the constraints are usually less severe, due to the impedance <strong>of</strong> the cables.<br />
Ignition <strong>of</strong> fluorescent tubes in groups implies one specific constraint. When a group<br />
<strong>of</strong> tubes is already switched on, the compensation capacitors in these tubes which<br />
are already energized participate in the inrush current at the moment <strong>of</strong> ignition <strong>of</strong><br />
a second group <strong>of</strong> tubes: they “amplify” the current peak in the control switch at the<br />
moment <strong>of</strong> ignition <strong>of</strong> the second group.<br />
t (s)<br />
t (s)<br />
N31<br />
© Schneider Electric - all rights reserved
N32<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
Fig. N44 : Electronic ballast<br />
4 Lighting circuits<br />
The table in Figure N43, resulting from measurements, specifies the magnitude <strong>of</strong><br />
the first current peak, for different values <strong>of</strong> prospective short-circuit current Isc. It is<br />
seen that the current peak can be multiplied by 2 or 3, depending on the number <strong>of</strong><br />
tubes already in use at the moment <strong>of</strong> connection <strong>of</strong> the last group <strong>of</strong> tubes.<br />
Number <strong>of</strong> tubes Number <strong>of</strong> tubes Inrush current peak (A)<br />
already in use connected Isc = 1,500 A Isc = 3,000 A Isc = 6,000 A<br />
0 14 233 250 320<br />
14 14 558 556 575<br />
28 14 608 607 624<br />
42 14 618 616 632<br />
Fig. N43 : Magnitude <strong>of</strong> the current peak in the control switch <strong>of</strong> the moment <strong>of</strong> ignition <strong>of</strong> a<br />
second group <strong>of</strong> tubes<br />
Nonetheless, sequential ignition <strong>of</strong> each group <strong>of</strong> tubes is recommended so as to<br />
reduce the current peak in the main switch.<br />
The most recent magnetic ballasts are known as “low-loss”. The magnetic circuit has<br />
been optimized, but the operating principle remains the same. This new generation<br />
<strong>of</strong> ballasts is coming into widespread use, under the influence <strong>of</strong> new regulations<br />
(European Directive, Energy Policy Act - USA).<br />
In these conditions, the use <strong>of</strong> electronic ballasts is likely to increase, to the<br />
detriment <strong>of</strong> magnetic ballasts.<br />
Fluorescent lamps with electronic ballast<br />
Electronic ballasts are used as a replacement for magnetic ballasts to supply power<br />
to fluorescent tubes (including compact fluorescent lamps) and discharge lamps.<br />
They also provide the “starter” function and do not need any compensation capacity.<br />
The principle <strong>of</strong> the electronic ballast (see Fig. N44) consists <strong>of</strong> supplying the lamp<br />
arc via an electronic device that generates a rectangular form AC voltage with a<br />
frequency between 20 and 60 kHz.<br />
Supplying the arc with a high-frequency voltage can totally eliminate the flicker<br />
phenomenon and strobe effects. The electronic ballast is totally silent.<br />
During the preheating period <strong>of</strong> a discharge lamp, this ballast supplies the lamp with<br />
increasing voltage, imposing an almost constant current. In steady state, it regulates<br />
the voltage applied to the lamp independently <strong>of</strong> any fluctuations in the line voltage.<br />
Since the arc is supplied in optimum voltage conditions, this results in energy<br />
savings <strong>of</strong> 5 to 10% and increased lamp service life. Moreover, the efficiency <strong>of</strong> the<br />
electronic ballast can exceed 93%, whereas the average efficiency <strong>of</strong> a magnetic<br />
device is only 85%.<br />
The power factor is high (> 0.9).<br />
The electronic ballast is also used to provide the light dimming function. Varying the<br />
frequency in fact varies the current magnitude in the arc and hence the luminous<br />
intensity.<br />
Inrush current<br />
The main constraint that electronic ballasts bring to line supplies is the high<br />
inrush current on switch-on linked to the initial load <strong>of</strong> the smoothing capacitors<br />
(see Fig. N45).<br />
Technology Max. inrush current Duration<br />
Rectifier with PFC 30 to 100 In y 1 ms<br />
Rectifier with choke 10 to 30 In y 5 ms<br />
Magnetic ballast y 13 In 5 to 10 ms<br />
Fig. N45 : Orders <strong>of</strong> magnitude <strong>of</strong> the inrush current maximum values, depending on the<br />
technologies used<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
4 Lighting circuits<br />
In reality, due to the wiring impedances, the inrush currents for an assembly <strong>of</strong> lamps<br />
is much lower than these values, in the order <strong>of</strong> 5 to 10 In for less than 5 ms.<br />
Unlike magnetic ballasts, this inrush current is not accompanied by an overvoltage.<br />
Harmonic currents<br />
For ballasts associated with high-power discharge lamps, the current drawn from<br />
the line supply has a low total harmonic distortion (< 20% in general and < 10% for<br />
the most sophisticated devices). Conversely, devices associated with low-power<br />
lamps, in particular compact fluorescent lamps, draw a very distorted current<br />
(see Fig. N46). The total harmonic distortion can be as high as 150%. In these<br />
conditions, the rms current drawn from the line supply equals 1.8 times the current<br />
corresponding to the lamp active power, which corresponds to a power factor <strong>of</strong> 0.55.<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
-0.2<br />
-0.4<br />
(A)<br />
-0.6<br />
0<br />
Fig. N46 : Shape <strong>of</strong> the current drawn by a compact fluorescent lamp<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
0.02<br />
In order to balance the load between the different phases, lighting circuits are usually<br />
connected between phases and neutral in a balanced way. In these conditions,<br />
the high level <strong>of</strong> third harmonic and harmonics that are multiple <strong>of</strong> 3 can cause an<br />
overload <strong>of</strong> the neutral conductor. The least favorable situation leads to a neutral<br />
current which may reach 3 times the current in each phase.<br />
Harmonic emission limits for electric or electronic systems are set by IEC standard<br />
61000-3-2. For simplification, the limits for lighting equipment are given here only for<br />
harmonic orders 3 and 5 which are the most relevant (see Fig. N47).<br />
Harmonic Active input Active input power y 25W<br />
order power > 25W one <strong>of</strong> the 2 sets <strong>of</strong> limits apply:<br />
% <strong>of</strong> fundamental % <strong>of</strong> fundamental Harmonic current relative<br />
current current to active power<br />
3 30 86 3.4 mA/W<br />
5 10 61 1.9 mA/W<br />
Fig. N47 : Maximum permissible harmonic current<br />
Leakage currents<br />
Electronic ballasts usually have capacitors placed between the power supply<br />
conductors and the earth. These interference-suppressing capacitors are responsible<br />
for the circulation <strong>of</strong> a permanent leakage current in the order <strong>of</strong> 0.5 to 1 mA per<br />
ballast. This therefore results in a limit being placed on the number <strong>of</strong> ballasts that<br />
can be supplied by a Residual Current Differential Safety Device (RCD).<br />
At switch-on, the initial load <strong>of</strong> these capacitors can also cause the circulation <strong>of</strong> a<br />
current peak whose magnitude can reach several amps for 10 µs. This current peak<br />
may cause unwanted tripping <strong>of</strong> unsuitable devices.<br />
t (s)<br />
N33<br />
© Schneider Electric - all rights reserved
N34<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
4 Lighting circuits<br />
High-frequency emissions<br />
Electronic ballasts are responsible for high-frequency conducted and radiated<br />
emissions.<br />
The very steep rising edges applied to the ballast output conductors cause current<br />
pulses circulating in the stray capacities to earth. As a result, stray currents circulate<br />
in the earth conductor and the power supply conductors. Due to the high frequency<br />
<strong>of</strong> these currents, there is also electromagnetic radiation. To limit these HF emissions,<br />
the lamp should be placed in the immediate proximity <strong>of</strong> the ballast, thus reducing<br />
the length <strong>of</strong> the most strongly radiating conductors.<br />
The different power supply modes (see Fig. N48)<br />
Technology Power supply mode Other device<br />
Standard incandescent Direct power supply Dimmer switch<br />
Halogen incandescent<br />
ELV halogen incandescent Transformer Electronic converter<br />
Fluorescent tube Magnetic ballast and starter Electronic ballast<br />
Electronic dimmer +<br />
ballast<br />
Compact fluorescent lamp Built-in electronic ballast<br />
Mercury vapor Magnetic ballast Electronic ballast<br />
High-pressure sodium<br />
Low-pressure sodium<br />
Metal halide<br />
Fig. N48 : Different power supply modes<br />
4.3 Constraints related to lighting devices and<br />
recommendations<br />
The current actually drawn by luminaires<br />
The risk<br />
This characteristic is the first one that should be defined when creating an<br />
<strong>installation</strong>, otherwise it is highly probable that overload protection devices will trip<br />
and users may <strong>of</strong>ten find themselves in the dark.<br />
It is evident that their determination should take into account the consumption <strong>of</strong><br />
all components, especially for fluorescent lighting <strong>installation</strong>s, since the power<br />
consumed by the ballasts has to be added to that <strong>of</strong> the tubes and bulbs.<br />
The solution<br />
For incandescent lighting, it should be remembered that the line voltage can be more<br />
than 10% <strong>of</strong> its nominal value, which would then cause an increase in the current<br />
drawn.<br />
For fluorescent lighting, unless otherwise specified, the power <strong>of</strong> the magnetic<br />
ballasts can be assessed at 25% <strong>of</strong> that <strong>of</strong> the bulbs. For electronic ballasts, this<br />
power is lower, in the order <strong>of</strong> 5 to 10%.<br />
The thresholds for the overcurrent protection devices should therefore be calculated<br />
as a function <strong>of</strong> the total power and the power factor, calculated for each circuit.<br />
Overcurrents at switch-on<br />
The risk<br />
The devices used for control and protection <strong>of</strong> lighting circuits are those such as<br />
relays, triac, remote-control switches, contactors or circuit-breakers.<br />
The main constraint applied to these devices is the current peak on energization.<br />
This current peak depends on the technology <strong>of</strong> the lamps used, but also on the<br />
<strong>installation</strong> characteristics (supply transformer power, length <strong>of</strong> cables, number <strong>of</strong><br />
lamps) and the moment <strong>of</strong> energization in the line voltage period. A high current<br />
peak, however fleeting, can cause the contacts on an electromechanical control<br />
device to weld together or the destruction <strong>of</strong> a solid state device with semiconductors.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
4 Lighting circuits<br />
Two solutions<br />
Because <strong>of</strong> the inrush current, the majority <strong>of</strong> ordinary relays are incompatible with<br />
lighting device power supply. The following recommendations are therefore usually<br />
made:<br />
b Limit the number <strong>of</strong> lamps to be connected to a single device so that their total<br />
power is less than the maximum permissible power for the device<br />
b Check with the manufacturers what operating limits they suggest for the devices.<br />
This precaution is particularly important when replacing incandescent lamps with<br />
compact fluorescent lamps<br />
By way <strong>of</strong> example, the table in Figure N49 indicates the maximum number <strong>of</strong><br />
compensated fluorescent tubes that can be controlled by different devices with<br />
16 A rating. Note that the number <strong>of</strong> controlled tubes is well below the number<br />
corresponding to the maximum power for the devices.<br />
Tube unit power Number <strong>of</strong> tubes Maximum number <strong>of</strong> tubes that can be<br />
requirement corresponding controlled by<br />
(W) to the power Contactors Remote Circuit-<br />
16 A x 230 V GC16 A control breakers<br />
CT16 A switches C60-16 A<br />
TL16 A<br />
18 204 15 50 112<br />
36 102 15 25 56<br />
58 63 10 16 34<br />
Fig. N49 : The number <strong>of</strong> controlled tubes is well below the number corresponding to the<br />
maximum power for the devices<br />
But a technique exists to limit the current peak on energization <strong>of</strong> circuits with<br />
capacitive behavior (magnetic ballasts with parallel compensation and electronic<br />
ballasts). It consists <strong>of</strong> ensuring that activation occurs at the moment when the line<br />
voltage passes through zero. Only solid state switches with semi-conductors <strong>of</strong>fer<br />
this possibility (see Fig. N50a). This technique has proved to be particularly useful<br />
when <strong>design</strong>ing new lighting circuits.<br />
More recently, hybrid technology devices have been developed that combine a solid<br />
state switch (activation on voltage passage through zero) and an electromechanical<br />
contactor short-circuiting the solid state switch (reduction <strong>of</strong> losses in the semiconductors)<br />
(see Fig. N50b).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
a b c<br />
Fig. N50 : “Standard” CT+ contactor [a], CT+ contactor with manual override, pushbutton for<br />
selection <strong>of</strong> operating mode and indicator lamp showing the active operating mode [b], and TL +<br />
remote-control switch [c] (Merlin Gerin brand)<br />
N35<br />
© Schneider Electric - all rights reserved
N36<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
Modular contactors and impulse relays do<br />
not use the same technologies. Their rating is<br />
determined according to different standards.<br />
For example, for a given rating, an impulse<br />
relay is more efficient than a modular<br />
contactor for the control <strong>of</strong> light fittings with<br />
a strong inrush current, or with a low power<br />
factor (non-compensated inductive circuit).<br />
Type<br />
<strong>of</strong> lamp<br />
Unit power<br />
and capacitance <strong>of</strong> power factor<br />
correction capacitor<br />
4 Lighting circuits<br />
Choice <strong>of</strong> relay rating according to lamp type<br />
b Figure 51 below shows the maximum number <strong>of</strong> light fittings for each relay,<br />
according to the type, power and configuration <strong>of</strong> a given lamp. As an indication, the<br />
total acceptable power is also mentioned.<br />
b These values are given for a 230 V circuit with 2 active conductors (single-phase<br />
phase/neutral or two-phase phase/phase). For 110 V circuits, divide the values in the<br />
table by 2.<br />
b To obtain the equivalent values for the whole <strong>of</strong> a 230 V three-phase circuit,<br />
multiply the number <strong>of</strong> lamps and the total acceptable power:<br />
v by 3 (1.73) for circuits without neutral;<br />
v by 3 for circuits with neutral.<br />
Note: The power ratings <strong>of</strong> the lamps most commonly used are shown in bold.<br />
Maximum number <strong>of</strong> light fittings for a single-phase circuit<br />
and maximum power output per circuit<br />
TL impulse relay CT contactor<br />
16 A 32 A 16 A 25 A 40 A 63 A<br />
Basic incandescent lamps<br />
LV halogen lamps<br />
Replacement mercury vapour lamps (without ballast)<br />
40 W 40 1500 W 106 4000 W 38 1550 W 57 2300 W 115 4600 W<br />
60 W 25 to 66 to 30 to 45 to 85 to 125<br />
75 W 20 1600 W 53 4200 W 25 2000 W 38 2850 W 70 5250 W 100<br />
100 W 16 42 19 28 50 73<br />
150 W 10 28 12 18 35 50<br />
200 W 8 21 10 14 26 37<br />
300 W 5 1500 W 13 4000 W 7 2100 W 10 3000 W 18 5500 W<br />
500 W 3 8 4 6 10 to 15<br />
1000 W 1 4 2 3 6 6000 W 8<br />
1500 W<br />
ELV 12 or 24 V halogen lamps<br />
1 2 1 2 4 5<br />
With ferromagnetic transformer 20 W 70 1350 W 180 3600 W 15 300 W 23 450 W 42 850 W<br />
50 W 28 to 74 to 10 to 15 to 27 to 42<br />
75 W 19 1450 W 50 3750 W 8 600 W 12 900 W 23 1950 W 35<br />
100 W 14 37 6 8 18 27<br />
With electronic transformer 20 W 60 1200 W 160 3200 W 62 1250 W 90 1850 W 182 3650 W<br />
50 W 25 to 65 to 25 to 39 to 76 to 114<br />
75 W 18 1400 W 44 3350 W 20 1600 W 28 2250 W 53 4200 W 78<br />
100 W<br />
Fluorescent tubes with starter and ferromagnetic ballast<br />
14 33 16 22 42 60<br />
1 tube<br />
15 W 83 1250 W 213 3200 W 22 330 W 30 450 W 70 1050 W<br />
without compensation (1)<br />
18 W<br />
20 W<br />
70<br />
62<br />
to<br />
1300 W<br />
186<br />
160<br />
to<br />
3350 W<br />
22<br />
22<br />
to<br />
850 W<br />
30<br />
30<br />
to<br />
1200 W<br />
70<br />
70<br />
to<br />
2400 W<br />
100<br />
100<br />
36 W 35 93 20 28 60 90<br />
40 W 31 81 20 28 60 90<br />
58 W 21 55 13 17 35 56<br />
65 W 20 50 13 17 35 56<br />
80 W 16 41 10 15 30 48<br />
115 W 11 29 7 10 20 32<br />
1 tube<br />
15 W 5 µF 60 900 W 160 2400 W 15 200 W 20 300 W 40 600 W<br />
with parallel compensation (2) 18 W 5 µF 50 133 15 to 20 to 40 to 60<br />
20 W 5 µF 45 120 15 800 W 20 1200 W 40 2400 W 60<br />
36 W 5 µF 25 66 15 20 40 60<br />
40 W 5 µF 22 60 15 20 40 60<br />
58 W 7 µF 16 42 10 15 30 43<br />
65 W 7 µF 13 37 10 15 30 43<br />
80 W 7 µF 11 30 10 15 30 43<br />
115 W 16 µF 7 20 5 7 14 20<br />
2 or 4 tubes<br />
2 x 18 W 56 2000 W 148 5300 W 30 1100 W 46 1650 W 80 2900 W<br />
with series compensation<br />
4 x 18 W<br />
2 x 36 W<br />
28<br />
28<br />
74<br />
74<br />
16<br />
16<br />
to<br />
1500 W<br />
24<br />
24<br />
to<br />
2400 W<br />
44<br />
44<br />
to<br />
3800 W<br />
68<br />
68<br />
2 x 58 W 17 45 10 16 27 42<br />
2 x 65 W 15 40 10 16 27 42<br />
2 x 80 W 12 33 9 13 22 34<br />
2 x 115 W 8 23 6 10 16 25<br />
Fluorescent tubes with electronic ballast<br />
1 or 2 tubes 18 W 80 1450 W 212 3800 W 74 1300 W 111 2000 W 222 4000 W<br />
36 W 40 to 106 to 38 to 58 to 117 to 176<br />
58 W 26 1550 W 69 4000 W 25 1400 W 37 2200 W 74 4400 W 111<br />
2 x 18 W 40 106 36 55 111 166<br />
2 x 36 W 20 53 20 30 60 90<br />
2 x 58 W 13 34 12 19 38 57<br />
Fig. N51 : Maximum number <strong>of</strong> light fittings for each relay, according to the type, power and configuration <strong>of</strong> a given lamp (Continued on opposite page)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
172 6900 W<br />
to<br />
7500 W<br />
25 7500 W<br />
to<br />
8000 W<br />
63 1250 W<br />
to<br />
2850 W<br />
275 5500 W<br />
to<br />
6000 W<br />
100 1500 W<br />
to<br />
3850 W<br />
60 900 W<br />
to<br />
3500 W<br />
123 4450 W<br />
to<br />
5900 W<br />
333 6000 W<br />
to<br />
6600 W
N - Characteristics <strong>of</strong> particular sources and loads<br />
Type<br />
<strong>of</strong> lamp<br />
Unit power<br />
and capacitance <strong>of</strong> power factor<br />
correction capacitor<br />
4 Lighting circuits<br />
Maximum number <strong>of</strong> light fittings for a single-phase circuit<br />
and maximum power output per circuit<br />
TL impulse relay CT contactor<br />
16 A 32 A 16 A 25 A 40 A 63 A<br />
Compact fluorescent lamps<br />
With external electronic ballast 5 W 240 1200 W 630 3150 W 210 1050 W 330 1650 W 670 3350 W not tested<br />
7 W 171 to 457 to 150 to 222 to 478 to<br />
9 W 138 1450 W 366 3800 W 122 1300 W 194 2000 W 383 4000 W<br />
11 W 118 318 104 163 327<br />
18 W 77 202 66 105 216<br />
26 W 55 146 50 76 153<br />
With integral electronic ballast 5 W 170 850 W 390 1950 W 160 800 W 230 1150 W 470 2350 W 710 3550 W<br />
(replacement for incandescent 7 W 121 to 285 to 114 to 164 to 335 to 514 to<br />
lamps)<br />
9 W 100 1050 W 233 2400 W 94 900 W 133 1300 W 266 2600 W 411 3950 W<br />
11 W 86 200 78 109 222 340<br />
18 W 55 127 48 69 138 213<br />
26 W 40 92 34 50 100 151<br />
High-pressure mercury vapour lamps with ferromagnetic ballast without ignitor<br />
Replacement high-pressure sodium vapour lamps with ferromagnetic ballast with integral ignitor (3)<br />
Without compensation (1) 50 W not tested,<br />
15 750 W 20 1000 W 34 1700 W 53 2650 W<br />
80 W infrequent use<br />
10 to 15 to 27 to 40 to<br />
125 / 110 W (3) 8 1000 W 10 1600 W 20 2800 W 28 4200 W<br />
250 / 220 W (3) 4 6 10 15<br />
400 / 350 W (3) 2 4 6 10<br />
700 W 1 2 4 6<br />
With parallel compensation (2) 50 W 7 µF 10 500 W 15 750 W 28 1400 W 43 2150 W<br />
80 W 8 µF 9 to 13 to 25 to 38 to<br />
125 / 110 W (3) 10 µF 9 1400 W 10 1600 W 20 3500 W 30 5000 W<br />
250 / 220 W (3) 18 µF 4 6 11 17<br />
400 / 350 W (3) 25 µF 3 4 8 12<br />
700 W 40 µF 2 2 5 7<br />
1000 W 60 µF 0 1 3 5<br />
Low-pressure sodium vapour lamps with ferromagnetic ballast with external ignitor<br />
Without compensation (1) 35 W not tested,<br />
5 270 W 9 320 W 14 500 W 24 850 W<br />
55 W infrequent use<br />
5 to 9 to 14 to 24 to<br />
90 W 3 360 W 6 720 W 9 1100 W 19 1800 W<br />
135 W 2 4 6 10<br />
180 W 2 4 6 10<br />
With parallel compensation (2) 35 W 20 µF 38 1350 W 102 3600 W 3 100 W 5 175 W 10 350 W 15 550 W<br />
55 W 20 µF 24 63 3 to 5 to 10 to 15 to<br />
90 W 26 µF 15 40 2 180 W 4 360 W 8 720 W 11 1100 W<br />
135 W 40 µF 10 26 1 2 5 7<br />
180 W 45 µF 7 18 1 2 4 6<br />
High-pressure sodium vapour lamps<br />
Metal-iodide lamps<br />
With ferromagnetic ballast with 35 W not tested,<br />
16 600 W 24 850 W 42 1450 W 64 2250 W<br />
external ignitor, without 70 W infrequent use<br />
8 12 to 20 to 32 to<br />
compensation (1)<br />
150 W 4 7 1200 W 13 2000 W 18 3200 W<br />
250 W 2 4 8 11<br />
400 W 1 3 5 8<br />
1000 W 0 1 2 3<br />
With ferromagnetic ballast with 35 W 6 µF 34 1200 W 88 3100 W 12 450 W 18 650 W 31 1100 W 50 1750 W<br />
external ignitor and parallel 70 W 12 µF 17 to 45 to 6 to 9 to 16 to 25 to<br />
compensation (2)<br />
150 W 20 µF 8 1350 W 22 3400 W 4 1000 W 6 2000 W 10 4000 W 15 6000 W<br />
250 W 32 µF 5 13 3 4 7 10<br />
400 W 45 µF 3 8 2 3 5 7<br />
1000 W 60 µF 1 3 1 2 3 5<br />
2000 W 85 µF 0 1 0 1 2 3<br />
With electronic ballast 35 W 38 1350 W 87 3100 W 24 850 W 38 1350 W 68 2400 W 102 3600 W<br />
70 W 29 to 77 to 18 to 29 to 51 to 76 to<br />
150 W 14 2200 W 33 5000 W 9 1350 W 14 2200 W 26 4000 W 40 6000 W<br />
(1) Circuits with non-compensated ferromagnetic ballasts consume twice as much current for a given lamp power output. This explains the small number <strong>of</strong> lamps in this<br />
configuration.<br />
(2) The total capacitance <strong>of</strong> the power factor correction capacitors in parallel in a circuit limits the number <strong>of</strong> lamps that can be controlled by a contactor. The total<br />
downstream capacitance <strong>of</strong> a modular contactor <strong>of</strong> rating 16, 25, 40 or 63 A should not exceed 75, 100, 200 or 300 µF respectively. Allow for these limits to calculate the<br />
maximum acceptable number <strong>of</strong> lamps if the capacitance values are different from those in the table.<br />
(3) High-pressure mercury vapour lamps without ignitor, <strong>of</strong> power 125, 250 and 400 W, are gradually being replaced by high-pressure sodium vapour lamps with integral<br />
ignitor, and respective power <strong>of</strong> 110, 220 and 350 W.<br />
Fig. N51 : Maximum number <strong>of</strong> light fittings for each relay, according to the type, power and configuration <strong>of</strong> a given lamp (Concluded)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
N37<br />
© Schneider Electric - all rights reserved
N38<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
Lamp<br />
power (W)<br />
Fig. N52 : Fluorescent tubes with electronic ballast - Vac = 230 V<br />
Protection <strong>of</strong> lamp circuits: Maximum number <strong>of</strong> lamps and MCB rating versus<br />
lamp type, unit power and MCB tripping curve<br />
During start up <strong>of</strong> discharge lamps (with their ballast), the inrush current drawn by<br />
each lamp may be in the order <strong>of</strong>:<br />
b 25 x circuit start current for the first 3 ms<br />
b 7 x circuit start current for the following 2 s<br />
For fluorescent lamps with High Frequency Electronic control ballast, the protective<br />
device ratings must cope with 25 x inrush for 250 to 350 µs.<br />
However due to the circuit resistance the total inrush current seen by the MCB is<br />
lower than the summation <strong>of</strong> all individual lamp inrush current if directly connected to<br />
the MCB.<br />
The tables below (see Fig. N52 to NXX) take into account:<br />
b Circuits cables have a length <strong>of</strong> 20 meters from distribution board to the first lamp<br />
and 7 meters between each additional fittings.<br />
b MCB rating is given to protect the lamp circuit in accordance with the cable cross<br />
section, and without unwanted tripping upon lamp starting.<br />
b MCB tripping curve (C = instantaneous trip setting 5 to 10 In, D = instantaneous<br />
trip setting 10 to 14 In).<br />
Number <strong>of</strong> lamps per circuit<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20<br />
MCB rating C & D tripping curve<br />
14/18 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
14 x2 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
14 x3 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10<br />
14 x4 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10<br />
18 x2 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
18 x4 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10<br />
21/24 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
21/24 x2 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
28 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
28 x2 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10<br />
35/36/39 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
35/36 x2 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10<br />
38/39 x2 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 10<br />
40/42 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
40/42 x2 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 10 16<br />
49/50 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
49/50 x2 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 16 16 16 16<br />
54/55 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10<br />
54/55 x2 6 6 6 6 6 6 6 6 10 10 10 10 10 10 16 16 16 16 16 16<br />
60 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10<br />
Lamp<br />
power (W)<br />
Number <strong>of</strong> lamps per circuit<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20<br />
MCB rating C & D tripping curve<br />
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
9 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
11 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
13 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
14 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
15 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
16 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
17 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
18 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
20 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
21 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
23 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
25 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10<br />
Fig. N53 : Compact fluorescent lamps - Vac = 230 V<br />
4 Lighting circuits<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
Lamp<br />
power (W)<br />
4 Lighting circuits<br />
Number <strong>of</strong> lamps per circuit<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20<br />
MCB rating C tripping curve<br />
50 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10<br />
80 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 16 16 16<br />
125 6 6 6 10 10 10 10 10 10 10 10 16 16 16 16 16 16 16 20 20<br />
250 6 10 10 16 16 16 16 16 16 20 20 25 25 25 32 32 32 32 40 40<br />
400 6 16 20 25 25 32 32 32 32 32 32 40 40 40 50 50 50 50 63 63<br />
1000 16 32 40 50 50 50 50 63 63 - - - - - - - - - - -<br />
MCB rating D tripping curve<br />
50 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10<br />
80 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 16 16 16<br />
125 6 6 6 6 6 6 10 10 10 10 10 16 16 16 16 16 16 16 20 20<br />
250 6 6 10 10 10 10 16 16 16 20 20 25 25 25 32 32 32 32 40 40<br />
400 6 10 16 16 20 20 25 25 25 32 32 40 40 40 50 50 50 50 63 63<br />
1000 10 20 25 32 40 40 50 63 63 - - - - - - - - - - -<br />
Fig. N54 : High pressure mercury vapour (with ferromagnetic ballast and PF correction) - Vac = 230 V<br />
Lamp<br />
power (W)<br />
Number <strong>of</strong> lamps per circuit<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20<br />
MCB rating C tripping curve<br />
Ferromagnetic ballast<br />
18 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
26 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
35/36 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
55 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10<br />
91 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 16 16 16 16<br />
131 6 6 6 10 10 10 10 10 10 10 10 10 16 16 16 16 16 16 16 20<br />
135 6 6 6 10 10 10 10 10 10 10 10 16 16 16 16 16 16 20 20 20<br />
180 6 6 10 10 10 10 10 10 16 16 16 16 20 20 20 20 25 25 25 25<br />
Electronic ballast<br />
36 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
55 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
66 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10<br />
91 6 6 6 6 6 10 10 10 10 10 10 10 10 10 10 10 16 16 16 16<br />
MCB rating D tripping curve<br />
Ferromagnetic ballast<br />
18 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
26 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
35/36 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
55 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10<br />
91 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 16 16 16 16<br />
131 6 6 6 6 6 6 6 10 10 10 10 10 16 16 16 16 16 16 16 20<br />
135 6 6 6 6 6 6 10 10 10 10 10 16 16 16 16 16 16 20 20 20<br />
180 6 6 6 6 10 10 10 10 16 16 16 16 20 20 20 20 25 25 25 25<br />
Electronic ballast<br />
36 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
55 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
66 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10<br />
91 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 16 16 16 16<br />
Fig. N55 : Low pressure sodium (with PF correction) - Vac = 230 V<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
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N40<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
Lamp<br />
power (W)<br />
Number <strong>of</strong> lamps per circuit<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20<br />
MCB rating C tripping curve<br />
Ferromagnetic ballast<br />
50 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10<br />
70 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 16 16 16<br />
100 6 6 6 6 6 6 6 6 10 10 10 10 10 16 16 16 16 16 16 16<br />
150 6 6 10 10 10 10 10 10 6 16 16 16 16 16 16 20 20 20 25 25<br />
250 6 10 16 16 16 20 20 20 20 20 20 25 25 25 32 32 32 32 40 40<br />
400 10 16 20 25 32 32 32 32 32 32 32 40 40 40 50 50 50 50 63 63<br />
1000 16 32 40 50 50 50 50 63 63 - - - - - - - - - - -<br />
Electronic ballast<br />
35 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
50 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10<br />
100 6 6 6 6 6 6 6 6 10 10 10 10 10 10 16 16 16 16 16 16<br />
MCB rating D tripping curve<br />
Ferromagnetic ballast<br />
50 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10<br />
70 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 16 16 16<br />
100 6 6 6 6 6 6 6 6 10 10 10 10 10 16 16 16 16 16 16 16<br />
150 6 6 6 6 6 10 10 10 10 16 16 16 16 16 16 20 20 20 25 25<br />
250 6 6 10 10 16 16 16 16 16 20 20 25 25 25 32 32 32 32 40 40<br />
400 6 10 16 16 20 20 25 25 25 32 32 40 40 40 50 50 50 50 63 63<br />
1000 10 20 32 32 40 40 50 63 63 - - - - - - - - - - -<br />
Electronic ballast<br />
35 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
50 6 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10<br />
100 6 6 6 6 6 6 6 6 10 10 10 10 10 10 16 16 16 16 16 16<br />
Fig. N56 : High pressure sodium (with PF correction) - Vac = 230 V<br />
Lamp<br />
power (W)<br />
Number <strong>of</strong> lamps per circuit<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20<br />
MCB rating C tripping curve<br />
Ferromagnetic ballast<br />
35 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
70 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 16 16 16<br />
150 6 6 10 10 10 10 10 10 10 16 16 16 16 16 16 20 20 20 25 25<br />
250 6 10 16 16 16 20 20 20 20 20 20 25 25 25 32 32 32 32 40 40<br />
400 6 16 20 25 25 32 32 32 32 32 32 40 40 40 50 50 50 50 63 63<br />
1000 16 32 40 50 50 50 50 63 63 63 63 63 63 63 63 63 63 63 63 63<br />
1800/2000 25 50 63 63 63 - - - - - - - - - - - - - - -<br />
Electronic ballast<br />
35 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
70 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 10<br />
150 6 6 6 10 10 10 10 10 10 10 16 16 16 16 16 16 16 20 20 20<br />
MCB rating D tripping curve<br />
Ferromagnetic ballast<br />
35 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
70 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 16 16 16<br />
150 6 6 6 6 6 10 10 10 10 16 16 16 16 16 16 20 20 20 25 25<br />
250 6 6 10 10 16 16 16 16 16 20 20 25 25 25 32 32 32 32 40 40<br />
400 6 10 16 16 20 20 25 25 25 32 32 40 40 40 50 50 50 50 63 63<br />
1000 16 20 32 32 40 50 50 63 63 - - - - - - - - - - -<br />
1800 16 32 40 50 63 63 - - - - - - - - - - - - - -<br />
2000 20 32 40 50 63 - - - - - - - - - - - - - - -<br />
Electronic ballast<br />
35 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6<br />
70 6 6 6 6 6 6 6 6 6 6 6 6 10 10 10 10 10 10 10 10<br />
150 6 6 6 6 6 6 6 10 10 10 16 16 16 16 16 16 16 20 20 20<br />
Fig. N57 : Metal halide (with PF correction) - Vac = 230 V<br />
Lamp<br />
power (W)<br />
Fig. N58 : Metal halide (with ferromagnetic ballast and PF correction) - Vac = 400 V<br />
4 Lighting circuits<br />
Number <strong>of</strong> lamps per circuit<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20<br />
MCB rating C tripping curve<br />
1800 16 32 40 50 50 50 50 63 63 - - - - - - - - - - -<br />
2000 16 32 40 50 50 50 50 63 63 - - - - - - - - - - -<br />
MCB rating D tripping curve<br />
1800 16 20 32 32 32 32 50 63 63 - - - - - - - - - - -<br />
2000 16 25 32 32 32 32 50 63 - - - - - - - - - - - -<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
Fig. N60 : s.i. residual current devices with immunity against<br />
impulse currents (Merlin Gerin brand)<br />
4 Lighting circuits<br />
Overload <strong>of</strong> the neutral conductor<br />
The risk<br />
In an <strong>installation</strong> including, for example, numerous fluorescent tubes with electronic<br />
ballasts supplied between phases and neutral, a high percentage <strong>of</strong> 3 rd harmonic<br />
current can cause an overload <strong>of</strong> the neutral conductor. Figure N59 below gives an<br />
overview <strong>of</strong> typical H3 level created by lighting.<br />
Lamp type Typical power Setting mode Typical H3 level<br />
Incandescend lamp 100 W Light dimmer 5 to 45 %<br />
with dimmer<br />
ELV halogen lamp 25 W Electronic ELV 5 %<br />
transformer<br />
Fluorescent tube 100 W Magnetic ballast 10 %<br />
< 25 W Electronic ballast 85 %<br />
> 25 W + PFC 30 %<br />
Discharge lamp 100 W Magnetic ballast 10 %<br />
Electrical ballast 30 %<br />
Fig. N59 : Overview <strong>of</strong> typical H3 level created by lighting<br />
The solution<br />
Firstly, the use <strong>of</strong> a neutral conductor with a small cross-section (half) should be<br />
prohibited, as requested by Installation standard IEC 60364, section 523–5–3.<br />
As far as overcurrent protection devices are concerned, it is necessary to provide<br />
4-pole circuit-breakers with protected neutral (except with the TN-C system for which<br />
the PEN, a combined neutral and protection conductor, should not be cut).<br />
This type <strong>of</strong> device can also be used for the breaking <strong>of</strong> all poles necessary to supply<br />
luminaires at the phase-to-phase voltage in the event <strong>of</strong> a fault.<br />
A breaking device should therefore interrupt the phase and Neutral circuit<br />
simultaneously.<br />
Leakage currents to earth<br />
The risk<br />
At switch-on, the earth capacitances <strong>of</strong> the electronic ballasts are responsible for<br />
residual current peaks that are likely to cause unintentional tripping <strong>of</strong> protection<br />
devices.<br />
Two solutions<br />
The use <strong>of</strong> Residual Current Devices providing immunity against this type <strong>of</strong> impulse<br />
current is recommended, even essential, when equipping an existing <strong>installation</strong><br />
(see Fig. N60).<br />
For a new <strong>installation</strong>, it is sensible to provide solid state or hybrid control devices<br />
(contactors and remote-control switches) that reduce these impulse currents<br />
(activation on voltage passage through zero).<br />
Overvoltages<br />
The risk<br />
As illustrated in earlier sections, switching on a lighting circuit causes a transient state<br />
which is manifested by a significant overcurrent. This overcurrent is accompanied by a<br />
strong voltage fluctuation applied to the load terminals connected to the same circuit.<br />
These voltage fluctuations can be detrimental to correct operation <strong>of</strong> sensitive loads<br />
(micro-computers, temperature controllers, etc.)<br />
The Solution<br />
It is advisable to separate the power supply for these sensitive loads from the lighting<br />
circuit power supply.<br />
Sensitivity <strong>of</strong> lighting devices to line voltage disturbances<br />
Short interruptions<br />
b The risk<br />
Discharge lamps require a relighting time <strong>of</strong> a few minutes after their power supply<br />
has been switched <strong>of</strong>f.<br />
b The solution<br />
Partial lighting with instantaneous relighting (incandescent lamps or fluorescent<br />
tubes, or “hot restrike” discharge lamps) should be provided if safety requirements so<br />
dictate. Its power supply circuit is, depending on current regulations, usually distinct<br />
from the main lighting circuit.<br />
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N - Characteristics <strong>of</strong> particular sources and loads<br />
4 Lighting circuits<br />
Voltage fluctuations<br />
b The risk<br />
The majority <strong>of</strong> lighting devices (with the exception <strong>of</strong> lamps supplied by electronic<br />
ballasts) are sensitive to rapid fluctuations in the supply voltage. These fluctuations<br />
cause a flicker phenomenon which is unpleasant for users and may even cause<br />
significant problems. These problems depend on both the frequency <strong>of</strong> variations<br />
and their magnitude.<br />
Standard IEC 61000-2-2 (“compatibility levels for low-frequency conducted<br />
disturbances”) specifies the maximum permissible magnitude <strong>of</strong> voltage variations as<br />
a function <strong>of</strong> the number <strong>of</strong> variations per second or per minute.<br />
These voltage fluctuations are caused mainly by high-power fluctuating loads (arc<br />
furnaces, welding machines, starting motors).<br />
b The solution<br />
Special methods can be used to reduce voltage fluctuations. Nonetheless, it is<br />
advisable, wherever possible, to supply lighting circuits via a separate line supply.<br />
The use <strong>of</strong> electronic ballasts is recommended for demanding applications<br />
(hospitals, clean rooms, inspection rooms, computer rooms, etc).<br />
Developments in control and protection equipment<br />
The use <strong>of</strong> light dimmers is more and more common. The constraints on ignition are<br />
therefore reduced and derating <strong>of</strong> control and protection equipment is less important.<br />
New protection devices adapted to the constraints on lighting circuits are being<br />
introduced, for example Merlin Gerin brand circuit-breakers and modular residual<br />
current circuit-breakers with special immunity, such as s.i. type ID switches and<br />
Vigi circuit-breakers. As control and protection equipment evolves, some now <strong>of</strong>fer<br />
remote control, 24-hour management, lighting control, reduced consumption, etc.<br />
4.4 Lighting <strong>of</strong> public areas<br />
Normal lighting<br />
Regulations governing the minimum requirements for buildings receiving the public in<br />
most European countries are as follows:<br />
b Installations which illuminates areas accessible to the public must be controlled<br />
and protected independently from <strong>installation</strong>s providing illumination to other areas<br />
b Loss <strong>of</strong> supply on a final lighting circuit (i.e. fuse blown or CB tripped) must not<br />
result in total loss <strong>of</strong> illumination in an area which is capable <strong>of</strong> accommodating more<br />
than 50 persons<br />
b Protection by Residual Current Devices (RCD) must be divided amongst several<br />
devices (i.e. more than on device must be used)<br />
Emergency lighting and other systems<br />
When we refer to emergency lighting, we mean the auxiliary lighting that is triggered<br />
when the standard lighting fails.<br />
Emergency lighting is subdivided as follows (EN-1838):<br />
Safety lighting<br />
It originates from the emergency lighting and is intended to provide lighting for people to<br />
evacuate an area safely or for those who try to fi nish a potentially dangerous operation before<br />
leaving the area. It is intended to illuminate the means <strong>of</strong> evacuation and ensure continuous<br />
visibility and ready usage in safety when standard or emergency lighting is needed.<br />
Safety lighting may be further subdivided as follows:<br />
Safety lighting for escape routes<br />
It originates from the safety lighting, and is<br />
intended to ensure that the escape means<br />
can be clearly identifi ed and used safely<br />
when the area is busy.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Anti-panic lighting in extended areas<br />
It originates from the safety lighting, and is<br />
intended to avoid panic and to provide the<br />
necessary lighting to allow people to reach<br />
a possible escape route area.<br />
Emergency lighting and safety signs for escape routes<br />
The emergency lighting and safety signs for escape routes are very important for all<br />
those who <strong>design</strong> emergency systems. Their suitable choice helps improve safety<br />
levels and allows emergency situations to be handled better.
N - Characteristics <strong>of</strong> particular sources and loads<br />
4 Lighting circuits<br />
Standard EN 1838 ("Lighting applications. Emergency lighting") gives some<br />
fundamental concepts concerning what is meant by emergency lighting for escape<br />
routes:<br />
"The intention behind lighting escape routes is to allow safe exit by the occupants,<br />
providing them with suffi cient visibility and directions on the escape route …"<br />
The concept referred to above is very simple:<br />
The safety signs and escape route lighting must be two separate things.<br />
Functions and operation <strong>of</strong> the luminaires<br />
The manufacturing specifi cations are covered by standard EN 60598-2-22,<br />
"Particular Requirements - Luminaires for Emergency Lighting", which must be read<br />
with EN 60598-1, "Luminaires – Part 1: <strong>General</strong> Requirements and Tests".<br />
Duration<br />
A basic requirement is to determine the duration required for the emergency lighting.<br />
<strong>General</strong>ly it is 1 hour but some countries may have different duration requirements<br />
according to statutory technical standards.<br />
Operation<br />
We should clarify the different types <strong>of</strong> emergency luminaires:<br />
b Non-maintained luminaires<br />
v The lamp will only switch on if there is a fault in the standard lighting<br />
v The lamp will be powered by the battery during failure<br />
v The battery will be automatically recharged when the mains power supply is<br />
restored<br />
b Maintained luminaires<br />
v The lamp can be switched on in continuous mode<br />
v A power supply unit is required with the mains, especially for powering the lamp,<br />
which can be disconnected when the area is not busy<br />
v The lamp will be powered by the battery during failure.<br />
Design<br />
The integration <strong>of</strong> emergency lighting with standard lighting must comply strictly with<br />
<strong>electrical</strong> system standards in the <strong>design</strong> <strong>of</strong> a building or particular place.<br />
All regulations and laws must be complied with in order to <strong>design</strong> a system which is<br />
up to standard (see Fig. N61).<br />
The main functions <strong>of</strong> an emergency lighting system<br />
when standard lighting fails are the following:<br />
b Provide sufficient emergency<br />
lighting along the escape paths<br />
so that people can safely find<br />
their ways to the exits.<br />
Fig. N61 : The main functions <strong>of</strong> an emergency lighting system<br />
European standards<br />
The <strong>design</strong> <strong>of</strong> emergency lighting systems is regulated by a number <strong>of</strong> legislative<br />
provisions that are updated and implemented from time to time by new<br />
documentation published on request by the authorities that deal with European and<br />
international technical standards and regulations.<br />
Each country has its own laws and regulations, in addition to technical standards<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
b Clearly show the escape<br />
route using clear signs.<br />
b Ensure that alarms and<br />
the fire safety equipment<br />
present along the way out<br />
are easily identifiable.<br />
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N - Characteristics <strong>of</strong> particular sources and loads<br />
4 Lighting circuits<br />
which govern different sectors. Basically they describe the places that must be<br />
provided with emergency lighting as well as its technical specifi cations. The<br />
<strong>design</strong>er's job is to ensure that the <strong>design</strong> project complies with these standards.<br />
EN 1838<br />
A very important document on a European level regarding emergency lighting is the<br />
Standard EN 1838, "Lighting applications. Emergency lighting".<br />
This standard presents specifi c requirements and constraints regarding the<br />
operation and the function <strong>of</strong> emergency lighting systems.<br />
CEN and CENELEC standards<br />
With the CEN (Comité Européen de Normalisation) and CENELEC standards<br />
(Comité Européen de Normalisation Electrotechnique), we are in a standardised<br />
environment <strong>of</strong> particular interest to the technician and the <strong>design</strong>er. A number<br />
<strong>of</strong> sections deal with emergencies. An initial distinction should be made between<br />
luminaire standards and <strong>installation</strong> standards.<br />
EN 60598-2-22 and EN-60598-1<br />
Emergency lighting luminaires are subject to European standard EN 60598-2-<br />
22, "Particular Requirements - Luminaires for Emergency Lighting", which is an<br />
integrative text (<strong>of</strong> specifi cations and analysis) <strong>of</strong> the Standard EN-60598-1,<br />
Luminaires – "Part 1: <strong>General</strong> Requirements and Tests".<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
The asynchronous (i.e. induction) motor is<br />
robust and reliable, and very widely used.<br />
95% <strong>of</strong> motors installed around the world are<br />
asynchronous. The protection <strong>of</strong> these motors<br />
is consequently a matter <strong>of</strong> great importance<br />
in numerous applications.<br />
td<br />
1 to 10s<br />
20 to<br />
30 ms<br />
t<br />
I" = 8 to 12 In<br />
Id = 5 to 8 In<br />
In = rated current <strong>of</strong> the motor<br />
In Id I"<br />
Fig. N62 : Direct on-line starting current characteristics <strong>of</strong> an<br />
induction motor<br />
I<br />
5 Asynchronous motors<br />
The consequence <strong>of</strong> an incorrectly protected motor can include the following:<br />
b For persons:<br />
v Asphyxiation due to the blockage <strong>of</strong> motor ventilation<br />
v Electrocution due to insulation failure in the motor<br />
v Accident due to non stopping <strong>of</strong> the motor following the failure <strong>of</strong> the control circuit<br />
in case <strong>of</strong> incorrect overcurrent protection<br />
b For the driven machine and the process<br />
v Shaft couplings and axles, etc, damaged due to a stalled rotor<br />
v Loss <strong>of</strong> production<br />
v Manufacturing time delayed<br />
b For the motor<br />
v Motor windings burnt out due to stalled rotor<br />
v Cost <strong>of</strong> dismantling and reinstalling or replacement <strong>of</strong> motor<br />
v Cost <strong>of</strong> repairs to the motor<br />
Therefore, the safety <strong>of</strong> persons and goods, and reliability and availability levels are<br />
highly dependant on the choice <strong>of</strong> protective equipment.<br />
In economic terms, the overall cost <strong>of</strong> failure must be considered. This cost<br />
is increasing with the size <strong>of</strong> the motor and with the difficulties <strong>of</strong> access and<br />
replacement. Loss <strong>of</strong> production is a further, and evidently important factor.<br />
Specific features <strong>of</strong> motor performance influence the power supply circuits required<br />
for satisfactory operation<br />
A motor power-supply circuit presents certain constraints not normally encountered<br />
in other (common) distribution circuits, owing to the particular characteristics, specific<br />
to motors, such as:<br />
b High start-up current (see Fig. N62) which is mostly reactive, and can therefore be<br />
the cause <strong>of</strong> important voltage drop<br />
b Number and frequency <strong>of</strong> start-up operations are generally high<br />
b The high start-up current means that motor overload protective devices must have<br />
operating characteristics which avoid tripping during the starting period<br />
5.1 Functions for the motor circuit<br />
Functions generally provided are:<br />
b Basic functions including:<br />
v Isolating facility<br />
v Motor control (local or remote)<br />
v Protection against short-circuits<br />
v Protection against overload<br />
b Complementary protections including:<br />
v Thermal protection by direct winding temperature measurement<br />
v Thermal protection by indirect winding temperature determination<br />
v Permanent insulation-resistance monitoring<br />
v Specific motor protection functions<br />
b Specific control equipment including:<br />
v Electromechanical starters<br />
v Control and Protective Switching devices (CPS)<br />
v S<strong>of</strong>t-start controllers<br />
v Variable speed drives<br />
Basic functions<br />
Isolating facility<br />
It is necessary to isolate the circuits, partially or totally, from their power supply<br />
network for satety <strong>of</strong> personnel during maintenance work. “Isolation” function is<br />
provided by disconnectors. This function can be included in other devices <strong>design</strong>ed<br />
to provide isolation such as disconnector/circuit-breaker.<br />
Motor control<br />
The motor control function is to make and break the motor current. In case <strong>of</strong> manual<br />
control, this function can be provided by motor-circuit-breakers or switches.<br />
In case <strong>of</strong> remote control, this function can be provided by contactors, starters or CPS.<br />
The control function can also be initiated by other means:<br />
b Overload protection<br />
b Complementary protection<br />
b Under voltage release (needed for a lot <strong>of</strong> machines)<br />
The control function can also be provided by specific control equipment.<br />
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N - Characteristics <strong>of</strong> particular sources and loads<br />
5 Asynchronous motors<br />
Protection against short-circuits<br />
b Phase-to-phase short-circuit<br />
This type <strong>of</strong> fault inside the machine is very rare. It is generally due to mechanical<br />
incident <strong>of</strong> the power supply cable <strong>of</strong> the motor.<br />
b Phase-to-earth short-circuit<br />
The deterioration <strong>of</strong> winding insulation is the main cause. The resulting fault current<br />
depends on the system <strong>of</strong> earthing. For the TN system, the resulting fault current is<br />
very high and in most cases the motor will be deteriorated. For the other systems <strong>of</strong><br />
earthing, protection <strong>of</strong> the motor can be achieved by earth fault protection.<br />
For short-circuit protection, it is recommended to pay special attention to avoid<br />
unexpected tripping during the starting period <strong>of</strong> the motor. The inrush current <strong>of</strong> a<br />
standard motor is about 6 to 8 times its rated current but during a fault the current<br />
can be as high as 15 times the rated current. So, the starting current must not be<br />
seen as a fault by the protection. In addition, a fault occuring in a motor circuit must<br />
not disturb any upstream circuit. As a consequence, discrimination/selectivity <strong>of</strong><br />
magnetic protections must be respected with all parts <strong>of</strong> the <strong>installation</strong>.<br />
Protection against overload<br />
Mechanical overloads due to the driven machine are the main origins <strong>of</strong> the overload<br />
for a motor application. They cause overload current and motor overheating. The life<br />
<strong>of</strong> the motor can be reduced and sometimes, the motor can be deteriorated. So, it is<br />
necessary to detect motor overload. This protection can be provided by:<br />
b Specific thermal overload relay<br />
b Specific thermal-magnetic circuit-breaker commonly referred to as “motor circuitbreaker”<br />
b Complementary protection (see below) like thermal sensor or electronic<br />
multifunction relay<br />
b Electronic s<strong>of</strong>t start controllers or variable speed drives (see below)<br />
Complementary protections<br />
b Thermal protection by direct winding temperature measurement<br />
Provided by thermal sensors incorporated inside the windings <strong>of</strong> the motor and<br />
associated relays.<br />
b Thermal protection by indirect winding temperature determination<br />
Provided by multifunction relays through current measurement and taking into<br />
account the characteristics <strong>of</strong> the motors (e.g.: thermal time constant).<br />
b Permanent insulation-resistance monitoring relays or residual current differential<br />
relays<br />
They provide detection and protection against earth leakage current and short-circuit<br />
to earth, allowing maintenance operation before destruction <strong>of</strong> the motor.<br />
b Specific motor protection functions<br />
Such as protection against too long starting period or stalled rotor, protection<br />
against unbalanced, loss or permutation <strong>of</strong> phases, earth fault protection, no load<br />
protection, rotor blocked (during start or after)…; pre alarm overheating indication,<br />
communication, can also be provided by multifunction relays.<br />
Specific control equipment<br />
b Electromechanical starters (star-delta, auto-transformer, rheostatic rotor<br />
starters,…)<br />
They are generally used for application with no load during the starting period (pump,<br />
fan, small centrifuge, machine-tool, etc.)<br />
v Advantages<br />
Good torque/current ratio; great reduction <strong>of</strong> inrush current.<br />
v Disadvantages<br />
Low torque during the starting period; no easy adjustment; power cut <strong>of</strong>f during the<br />
transition and transient phenomenon; 6 motor connection cables needed.<br />
b Control and Protective Switching devices (CPS)<br />
They provide all the basic functions listed before within a single unit and also some<br />
complementary functions and the possibility <strong>of</strong> communication. These devices also<br />
provide continuity <strong>of</strong> service in case <strong>of</strong> short-circuit.<br />
b S<strong>of</strong>t-start controllers<br />
Used for applications with pump, fan, compressor, conveyor.<br />
v Advantages<br />
Reduced inrush current, voltage drop and mechanical stress during the motor start;<br />
built-in thermal protection; small size device; possibility <strong>of</strong> communication<br />
v Disadvantages<br />
Low torque during the starting period; thermal dissipation.<br />
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N - Characteristics <strong>of</strong> particular sources and loads<br />
5 Asynchronous motors<br />
b Variable speed drives<br />
They are used for applications with pump, fan, compressor, conveyor, machine with<br />
high load torque, machine with high inertia.<br />
v Advantages<br />
Continuous speed variation (adjustment typically from 2 to 130% <strong>of</strong> nominal speed),<br />
overspeed is possible; accurate control <strong>of</strong> acceleration and deceleration; high<br />
torque during the starting and stopping periods; low inrush current, built-in thermal<br />
protection, possibility <strong>of</strong> communication.<br />
v Disadvantages<br />
Thermal dissipation, volume, cost.<br />
5.2 Standards<br />
The motor control and protection can be achieved in different way:<br />
b By using an association <strong>of</strong> a SCPD (Short-Circuit-Protective-Device) and<br />
electromechanical devices such as<br />
v An electromechanical starters fulfilling the standard IEC 60947-4-1<br />
v A semiconductor starter fulfilling the standard IEC 60947-4-2<br />
v A variable speed drives fulfilling the standard series IEC 61800<br />
b By using a CPS, single device covering all the basic functions, and fulfilling the<br />
standard IEC 60947-6-2<br />
In this document, only the motor circuits including association <strong>of</strong> electromechanical<br />
devices such as, starters and protection against short-circuit, are considered. The<br />
devices meeting the standard 60947-6-2, the semiconductor starters and the variable<br />
speed drives will be considered only for specific points.<br />
A motor circuit will meet the <strong>rules</strong> <strong>of</strong> the IEC 60947-4-1 and mainly:<br />
b The co-ordination between the devices <strong>of</strong> the motor circuit<br />
b The tripping class <strong>of</strong> the thermal relays<br />
b The category <strong>of</strong> utilization <strong>of</strong> the contactors<br />
b The insulation co-ordination<br />
Note: The first and last points are satisfied inherently by the devices meeting the<br />
IEC 60947-6-2 because they provide a continuity <strong>of</strong> service.<br />
Standardization <strong>of</strong> the association circuit-breaker + contactor<br />
+ thermal relay<br />
Utilization category <strong>of</strong> the contactors<br />
Standard IEC 60947-4-1 gives utilization categories which considerably facilitate<br />
the choice <strong>of</strong> a suitable contactor for a given service duty. The utilization categories<br />
advise on:<br />
b A range <strong>of</strong> functions for which the contactor must be adapted<br />
b The required current breaking and making capabilities<br />
b Standard values for on-load durability tests, according to the utilization category.<br />
Figure N63 gives some typical examples <strong>of</strong> the utilization categories covered.<br />
Utilization category Application characteristics<br />
AC-1 Non-inductive (or slightly inductive) loads:<br />
cos ϕ u 0.95 (heating, distribution)<br />
AC-2 Starting and switching <strong>of</strong>f <strong>of</strong> slip-ring motors<br />
AC-3 Cage motors: Starting, and switching <strong>of</strong>f motors<br />
during running<br />
AC-4 Cage motors: Starting, plugging, inching<br />
Fig. N63 : Utilization categories for contactors<br />
Note: These utilization categories are adapted to the devices meeting the other<br />
standards. For example AC-3 becomes AC-53 for the semiconductor starters<br />
(IEC 60947-4-2) and becomes AC-43 for CPS’s (IEC 60947-6-2).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
N47<br />
© Schneider Electric - all rights reserved
N48<br />
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N - Characteristics <strong>of</strong> particular sources and loads<br />
Among the many possible methods <strong>of</strong><br />
protecting a motor, the association <strong>of</strong> a<br />
circuit breaker + contactor + thermal relay (1)<br />
provides many advantages<br />
(1) The combination <strong>of</strong> a contactor with a thermal relay is<br />
commonly referred to as a “discontactor”<br />
5 Asynchronous motors<br />
The types <strong>of</strong> co-ordination<br />
For each association <strong>of</strong> devices, a type <strong>of</strong> co-ordination is given, according to the<br />
state <strong>of</strong> the constituant parts following a circuit-breaker trip out on fault, or the<br />
opening <strong>of</strong> a contactor on overload.<br />
The standard IEC 947-4-1 defines two types <strong>of</strong> co-ordination, type 1 and type 2,<br />
which set maximum allowable limits <strong>of</strong> deterioration <strong>of</strong> switchgear, in case <strong>of</strong> shortcircuit.<br />
Whatever the type <strong>of</strong> co-ordination, it is required that the contactor or the starter must<br />
never present a danger for the personnel and for the <strong>installation</strong>. The specificities <strong>of</strong><br />
each type are:<br />
b Type 1<br />
Deterioration <strong>of</strong> the starter is acceptable after a short-circuit and the operation <strong>of</strong> the<br />
starter may be recovered after reparing or replacing some parts.<br />
b Type 2<br />
Burning and the risk <strong>of</strong> welding <strong>of</strong> the contacts <strong>of</strong> the contactor are the only risks<br />
allowed.<br />
Which type to choose?<br />
The type <strong>of</strong> co-ordination to adopt depends on the parameters <strong>of</strong> exploitation and must<br />
be chosen to satisfy (optimally) the needs <strong>of</strong> the user and the cost <strong>of</strong> <strong>installation</strong>.<br />
b Type 1<br />
v Qualified maintenance service<br />
v Volume and cost <strong>of</strong> switchgear reduced<br />
v May not be suitable for further service without repair or replacement <strong>of</strong> parts after<br />
a short-circuit<br />
b Type 2<br />
v Only light maintenance measures for further use after a short-circuit<br />
5.3 Applications<br />
The control and protection <strong>of</strong> a motor can consist <strong>of</strong> one, two, three or four different<br />
devices which provide one or several functions.<br />
In the case <strong>of</strong> the combination <strong>of</strong> several devices, co-ordination between them<br />
is essential in order to provide optimized protection <strong>of</strong> the motor application.<br />
To protect a motor circuit, many parameters must be taken into account. They<br />
depend on:<br />
b The application (type <strong>of</strong> driven machine, safety <strong>of</strong> operation, number <strong>of</strong> operations,<br />
etc.)<br />
b The continuity performance requested by the application<br />
b The standards to be enforced to provide security and safety.<br />
The <strong>electrical</strong> functions to be provided are quite different:<br />
b Start, normal operation and stop without unexpected tripping while maintaining<br />
control requirements, number <strong>of</strong> operations, durability and safety requirements<br />
(emergency stops), as well as circuit and motor protection, disconnection (isolation)<br />
for safety <strong>of</strong> personnel during maintenance work.<br />
Basic protection schemes: circuit-breaker + contactor<br />
+ thermal relay<br />
Avantages<br />
The combination <strong>of</strong> devices facilitates <strong>installation</strong> work, as well as operation and<br />
maintenance, by:<br />
b The reduction <strong>of</strong> the maintenance work load: the circuit-breaker avoids the need to<br />
replace blown fuses and the necessity <strong>of</strong> maintaining a stock (<strong>of</strong> different sizes and<br />
types)<br />
b Better continuity performance: the <strong>installation</strong> can be re-energized immediately<br />
following the elimination <strong>of</strong> a fault and after checking <strong>of</strong> the starter<br />
b Additional complementary devices sometimes required on a motor circuit are<br />
easily accomodated<br />
b Tripping <strong>of</strong> all three phases is assured (thereby avoiding the possibility <strong>of</strong> “single<br />
phasing”)<br />
b Full load current switching possibility (by circuit-breaker) in the event <strong>of</strong> contactor<br />
failure, e.g. contact welding<br />
b Interlocking<br />
b Diverse remote indications<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
Circuit<br />
breaker<br />
Magnetic<br />
relay<br />
Contactor<br />
Thermal<br />
relay<br />
Cable<br />
Motor<br />
End <strong>of</strong><br />
start-up<br />
period<br />
1 to<br />
10 s<br />
20 to<br />
30 ms<br />
Fig. N64 : Tripping characteristics <strong>of</strong> a circuit-breaker + contactor + thermal relay (1)<br />
(1) In the majority <strong>of</strong> cases, short-circuit faults occur at the<br />
motor, so that the current is limited by the cable and the wiring<br />
<strong>of</strong> the starter and are called impedant short-circuits<br />
t<br />
In Is<br />
5 Asynchronous motors<br />
b Better protection for the starter in case <strong>of</strong> overcurrent and in particular for impedant<br />
short-circuit (1) corresponding to currents up to about 30 times In <strong>of</strong> motor (see Fig. N64).<br />
b Possibility <strong>of</strong> adding RCD:<br />
v Prevention <strong>of</strong> risk <strong>of</strong> fire (sensitivity 500 mA)<br />
v Protection against destruction <strong>of</strong> the motor (short-circuit <strong>of</strong> laminations) by the<br />
early detection <strong>of</strong> earth fault currents (sensitivity 300 mA to 30 A)<br />
I" magn.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
1.05 to 1.20 In<br />
Operating curve<br />
<strong>of</strong> thermal relay<br />
Cable thermal withstand limit<br />
Limit <strong>of</strong> thermal relay constraint<br />
Short circuit current breaking capacity<br />
<strong>of</strong> the association (CB + contactor)<br />
Operating curve <strong>of</strong> the<br />
MA type circuit breaker<br />
I<br />
Short circuit current breaking capacity<br />
<strong>of</strong> the CB<br />
Conclusion<br />
The combination <strong>of</strong> a circuit-breaker + contactor + thermal relay for the control and<br />
protection <strong>of</strong> motor circuits is eminently appropriate when:<br />
b The maintenance service for an <strong>installation</strong> is reduced, which is generally the case<br />
in tertiary and small and medium sized industrial sites<br />
b The job specification calls for complementary functions<br />
b There is an operational requirement for a load breaking facility in the event <strong>of</strong> need<br />
<strong>of</strong> maintenance.<br />
Key points in the successful combination <strong>of</strong> a circuit-breaker<br />
and a discontactor<br />
Standards define precisely the elements which must be taken into account to<br />
achieve a correct coordination <strong>of</strong> type 2:<br />
b Absolute compatibility between the thermal relay <strong>of</strong> the discontactor and the<br />
magnetic trip <strong>of</strong> the circuit-breaker. In Figure N65 the thermal relay is protected if<br />
its limit boundary for thermal withstand is placed to the right <strong>of</strong> the circuit-breaker<br />
magnetic trip characteristic curve. In the case <strong>of</strong> a motor control circuit-breaker<br />
incorporating both magnetic and thermal relay devices, coordination is provided by<br />
<strong>design</strong>.<br />
Compact<br />
type MA<br />
Icc ext.<br />
t<br />
2<br />
1<br />
1 Operating curve <strong>of</strong> the MA type circuit breaker<br />
2 Operating curve <strong>of</strong> thermal relay<br />
3 Limit <strong>of</strong> thermal relay constraint<br />
Fig. N65 : The thermal-withstand limit <strong>of</strong> the thermal relay must be to the right <strong>of</strong> the CB<br />
magnetic-trip characteristic<br />
3<br />
I<br />
N49<br />
© Schneider Electric - all rights reserved
N50<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
It is not possible to predict the short-circuit<br />
current-breaking capacity <strong>of</strong> a circuit-breaker<br />
+ contactor combination. Only laboratory tests<br />
by manufacturers allow to do it. So, Schneider<br />
Electric can give table with combination <strong>of</strong><br />
Multi 9 and Compact type MA circuit-breakers<br />
with different types <strong>of</strong> starters<br />
M<br />
Fig. N66 : Circuit-breaker and contactor mounted side by side<br />
M<br />
Fig. N67 : Circuit-breaker and contactor mounted separately<br />
Fig. N68 : Overheating protection by thermal sensors<br />
5 Asynchronous motors<br />
b The overcurrent breaking capability <strong>of</strong> the contactor must be greater than the<br />
current corresponding to the setting <strong>of</strong> the circuit-breaker magnetic trip relay.<br />
b When submitted to a short-circuit current, the contactor and its thermal relay must<br />
perform in accordance with the requirements corresponding to the specified type <strong>of</strong><br />
co-ordination.<br />
Short-circuit current-breaking capacity <strong>of</strong> a circuit-breaker<br />
+ contactor combination<br />
At the selection stage, the short-circuit current-breaking capacity which must be<br />
compared to the prospective short-circuit current is:<br />
b Either, that <strong>of</strong> the circuit-breaker + contactor combination if the circuit-breaker<br />
and the contactor are physically close together (see Fig. N66) (same drawer or<br />
compartment <strong>of</strong> a motor control cabinet). A short-circuit downstream <strong>of</strong> the<br />
combination will be limited to some extent by the impedances <strong>of</strong> the contactor and<br />
the thermal relay. The combination can therefore be used on a circuit for which<br />
the prospective short-circuit current level exceeds the rated short-circuit currentbreaking<br />
capacity <strong>of</strong> the circuit-breaker. This feature very <strong>of</strong>ten presents a significant<br />
economic advantage<br />
b Or that <strong>of</strong> the circuit-breaker only, for the case where the contactor is separated<br />
(see Fig. N67) with the risk <strong>of</strong> short-circuit between the contactor and the circuitbreaker.<br />
Choice <strong>of</strong> instantaneous magnetic-trip relay for the circuitbreaker<br />
The operating threshold must never be less than 12 In for this relay, in order to avoid<br />
unexpected tripping due to the first current peak during motor starting.<br />
Complementary protections<br />
Complementary protections are:<br />
b Thermal sensors in the motor (windings, bearings, cooling-air ducts, etc.)<br />
b Multifunction protections (association <strong>of</strong> functions)<br />
b Insulation-failure detection devices on running or stationary motor<br />
Thermal sensors<br />
Thermal sensors are used to detect abnormal temperature rise in the motor by direct<br />
measurement. The thermal sensors are generally embedded in the stator windings<br />
(for LV motors), the signal being processed by an associated control device acting to<br />
trip the contactor or the circuit-breaker (see Fig. N68).<br />
Mutifunction motor protection relay<br />
The multifunction relay, associated with a number <strong>of</strong> sensors and indication modules,<br />
provides protection for motor and also for some functions, protection <strong>of</strong> the driven<br />
machine such as:<br />
b Thermal overload<br />
b Stalled rotor, or starting period too long<br />
b Overheating<br />
b Unbalanced phase current, loss <strong>of</strong> one phase, inverse rotation<br />
b Earth fault (by RCD)<br />
b Running at no-load, blocked rotor on starting<br />
The avantages are essentially:<br />
b A comprehensive protection, providing a reliable, high performance and permanent<br />
monitoring/control function<br />
b Efficient monitoring <strong>of</strong> all motor-operating schedules<br />
b Alarm and control indications<br />
b Possibility <strong>of</strong> communication via communication buses<br />
Example: Telemecanique LT6 relay with permanent monitoring/control function<br />
and communication by bus, or multifunction control unit LUCM and communication<br />
module for TeSys model U.<br />
Preventive protection <strong>of</strong> stationary motors<br />
This protection concerns the monitoring <strong>of</strong> the insulation resistance level <strong>of</strong> a<br />
stationary motor, thereby avoiding the undesirable consequences <strong>of</strong> insulation failure<br />
during operation such as:<br />
b Failure to start or to perform correctly for motor used on emergency systems<br />
b Loss <strong>of</strong> production<br />
This type <strong>of</strong> protection is essential for emergency systems motors, especially when<br />
installed in humid and/or dusty locations. Such protection avoids the destruction <strong>of</strong><br />
a motor by short-circuit to earth during starting (one <strong>of</strong> the most frequently-occuring<br />
incidents) by giving a warning informing that maintenance work is necessary to<br />
restore the motor to a satisfactory operationnal condition.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
N - Characteristics <strong>of</strong> particular sources and loads<br />
5 Asynchronous motors<br />
Example <strong>of</strong> application:<br />
Motors driving pumps for “sprinklers” fire-protection systems or irrigation pumps for<br />
seasonal operation.<br />
A Vigilohm SN21 (Merlin Gerin) monitors the insulation <strong>of</strong> a motor, and signals<br />
audibly and visually any abnormal reduction <strong>of</strong> the insulation resistance level.<br />
Furthermore, this relay can prevent any attempt to start the motor, if necessary<br />
(see Fig. N69).<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
SM21<br />
MERLIN GERIN<br />
Fig. N69 : Preventive protection <strong>of</strong> stationary motors<br />
Fig. N70 : Example using relay RH99M<br />
RH99M<br />
SM20<br />
M E R L IN G E R IN<br />
IN OUT<br />
Limitative protections<br />
Residual current diffential protective devices (RCDs) can be very sensitive and<br />
detect low values <strong>of</strong> leakage current which occur when the insulation to earth <strong>of</strong> an<br />
<strong>installation</strong> deteriorates (by physical damage, contamination, excessive humidity,<br />
and so on). Some versions <strong>of</strong> RCDs, with dry contacts, specially <strong>design</strong>ed for such<br />
applications, provide the following:<br />
b To avoid the destruction <strong>of</strong> a motor (by perforation and short-circuiting <strong>of</strong> the<br />
laminations <strong>of</strong> the stator) caused by an eventual arcing fault to earth. This protection<br />
can detect incipient fault conditions by operating at leakage currents in the range <strong>of</strong><br />
300 mA to 30 A, according to the size <strong>of</strong> the motor (approx sensitivity: 5% In)<br />
b To reduce the risk <strong>of</strong> fire: sensitivity y 500 mA<br />
For example, RH99M relay (Merlin Gerin) provides (see Fig. N70):<br />
b 5 sensitivities (0.3; 1; 3; 10; 30 A)<br />
b Possibility <strong>of</strong> discrimination or to take account <strong>of</strong> particular operation by virtue <strong>of</strong> 3<br />
possible time delays (0, 90, 250 ms)<br />
b Automatic breaking if the circuit from the current transformer to the relay is broken<br />
b Protection against unwanted trippings<br />
b Protection against DC leakage currents (type A RCD)<br />
N51<br />
© Schneider Electric - all rights reserved
N52<br />
© Schneider Electric - all rights reserved<br />
N - Characteristics <strong>of</strong> particular sources and loads<br />
5 Asynchronous motors<br />
The importance <strong>of</strong> limiting the voltage drop at the motor terminals during<br />
start-up<br />
In order to have a motor starting and accelerating to its normal speed in the<br />
appropriate time, the torque <strong>of</strong> the motor must exceed the load torque by at least<br />
70%. However, the starting current is much higher than the full-load current <strong>of</strong><br />
the motor. As a result, if the voltage drop is very high, the motor torque will be<br />
excessively reduced (motor torque is proportional to U 2 ) and it will result, for extreme<br />
case, in failure to start.<br />
Example:<br />
b With 400 V maintained at the terminals <strong>of</strong> a motor, its torque would be 2.1 times<br />
that <strong>of</strong> the load torque<br />
b For a voltage drop <strong>of</strong> 10% during start-up, the motor torque would be<br />
2.1 x 0.9 2 = 1.7 times the load torque, and the motor would accelerate to its rated<br />
speed normally<br />
b For a voltage drop <strong>of</strong> 15% during start-up, the motor torque would be<br />
2.1 x 0.85 2 = 1.5 times the load torque, so that the motor starting time would<br />
be longer than normal<br />
In general, a maximum allowable voltage drop <strong>of</strong> 10% is recommended during<br />
start-up <strong>of</strong> the motor.<br />
5.4 Maximum rating <strong>of</strong> motors installed for<br />
consumers supplied at LV<br />
The disturbances caused on LV distribution networks during the start-up <strong>of</strong> large<br />
direct-on-line AC motors can cause considerable nuisance to neighbouring<br />
consumers, so that most power-supply utilities have strict <strong>rules</strong> intended to limit such<br />
disturbances to tolerable levels. The amount <strong>of</strong> disturbance created by a given motor<br />
depends on the “strength” <strong>of</strong> the network, i.e. on the short-circuit fault level at the<br />
point concerned. The higher the fault level, the “stronger” the system and the lower<br />
the disturbance (principally voltage drop) experienced by neibouring consumers. For<br />
distribution networks in many countries, typical values <strong>of</strong> maximum allowable starting<br />
currents and corresponding maximum power ratings for direct-on-line motors are<br />
shown in Figures N71 and N72 below.<br />
Type <strong>of</strong> motor Location Maximum starting current (A)<br />
Overhead-line network Underground-cable network<br />
Single phase Dwellings 45 45<br />
Others 100 200<br />
Three phase Dwellings 60 60<br />
Others 125 250<br />
Fig. N71 : Maximum permitted values <strong>of</strong> starting current for direct-on-line LV motors (230/400 V)<br />
Location Type <strong>of</strong> motor<br />
Single phase 230 V Three phase 400 V<br />
(kW) Direct-on-line starting Other methods<br />
at full load (kW) <strong>of</strong> starting (kW)<br />
Dwellings 1.4 5.5 11<br />
Others Overhead 3 11 22<br />
line network<br />
Underground 5.5 22 45<br />
cable network<br />
Fig. N72 : Maximum permitted power ratings for LV direct-on-line starting motors<br />
Since, even in areas supplied by one power utility only, “weak” areas <strong>of</strong> the network<br />
exist as well as “strong” areas, it is always advisable to secure the agreement <strong>of</strong> the<br />
power supplier before acquiring the motors for a new project.<br />
Other (but generally more costly) alternative starting arrangements exist, which<br />
reduce the large starting currents <strong>of</strong> direct-on-line motors to acceptable levels; for<br />
example, star-delta starters, slip-ring motor, “s<strong>of</strong>t start” electronic devices, etc.<br />
5.5 Reactive-energy compensation (power-factor<br />
correction)<br />
The method to correct the power factor is indicated in chapter L.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
2<br />
3<br />
<strong>Chapter</strong> P<br />
Residential and other special<br />
locations<br />
Contents<br />
Residential and similar premises P<br />
1.1 <strong>General</strong> P1<br />
1.2 Distribution boards components P1<br />
1.3 Protection <strong>of</strong> people P4<br />
1.4 Circuits P6<br />
1.5 Protection against overvoltages and lightning P7<br />
Bathrooms and showers P8<br />
2.1 Classification <strong>of</strong> zones P8<br />
2.2 Equipotential bonding P11<br />
2.3 Requirements prescribed for each zone P11<br />
Recommendations applicable to special <strong>installation</strong>s P 2<br />
and locations<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
P<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
P2<br />
P - Residential and other special locations<br />
Electrical <strong>installation</strong>s for residential premises<br />
need a high standard <strong>of</strong> safety and reliability<br />
The power distribution utility connects the LV<br />
neutral point to its MV/LV distribution tranformer<br />
to earth.<br />
All LV <strong>installation</strong>s must be protected by RCDs.<br />
All exposed conductive parts must be bonded<br />
together and connected to the earth.<br />
The quality <strong>of</strong> <strong>electrical</strong> equipment used in<br />
residential premises is commonly ensured by a<br />
mark <strong>of</strong> conformity situated on the front <strong>of</strong> each<br />
item<br />
Fig. P1 : Presentation <strong>of</strong> realizable functions on a consumer unit<br />
Enclosure<br />
Residential and similar premises<br />
. <strong>General</strong><br />
Related standards<br />
Most countries have national regulations and-or standards governing the <strong>rules</strong><br />
to be strictly observed in the <strong>design</strong> and realization <strong>of</strong> <strong>electrical</strong> <strong>installation</strong>s for<br />
residential and similar premises. The relevant international standard is the publication<br />
IEC 60364.<br />
The power network<br />
The vast majority <strong>of</strong> power distribution utilities connect the low voltage neutral point<br />
<strong>of</strong> their MV/LV distribution transformers to earth.<br />
The protection <strong>of</strong> persons against electric shock therefore depends, in such case, on<br />
the principle discussed in chapter F. The measures required depend on whether the<br />
TT, TN or IT scheme <strong>of</strong> earthing is adopted.<br />
RCDs are essential for TT and IT earthed <strong>installation</strong>s. For TN <strong>installation</strong>s, high<br />
speed overcurrent devices or RCDs may provide protection against direct contact<br />
<strong>of</strong> the <strong>electrical</strong> circuits. To extend the protection to flexible leads beyond the fixed<br />
socket outlets and to ensure protection against fires <strong>of</strong> <strong>electrical</strong> origin RCDs shall<br />
be installed.<br />
.2 Distribution boards components (see Fig. P )<br />
Distribution boards (generally only one in residential premises) usually include<br />
the meter(s) and in some cases (notably where the supply utilities impose a TT<br />
earthing system and/or tariff conditions which limit the maximum permitted current<br />
consumption) an incoming supply differential circuit-breaker which includes an<br />
overcurrent trip. This circuit-breaker is freely accessible to the consumer.<br />
Service connection<br />
Lightning protection<br />
Overcurrent<br />
protection<br />
and isolation<br />
Protection against<br />
direct and indirect<br />
contact,<br />
and protection<br />
against fire<br />
Remote control<br />
Energy management<br />
Incoming-supply<br />
circuit breaker<br />
MCB phase and neutral<br />
Remote control switch<br />
TL 16 A<br />
Programmable thermostat<br />
THP<br />
Programmable time switch<br />
IHP<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Differential<br />
MCB<br />
Distribution board<br />
Combi surge arrester<br />
Differential load<br />
switch<br />
Load shedding switch<br />
CDSt<br />
Contactors, <strong>of</strong>f-peak<br />
or manual control CT
P - Residential and other special locations<br />
Fig. P3 : Incoming-supply circuit-breaker<br />
Fig. P4 : Control and distribution board<br />
If, in a TT scheme, the value <strong>of</strong> 80 Ω for the<br />
resistance <strong>of</strong> the electrode can not be met then,<br />
30 mA RCDs must be installed to take over the<br />
function <strong>of</strong> the earth leakage protection <strong>of</strong> the<br />
incoming supply circuit-breaker<br />
Residential and similar premises<br />
On <strong>installation</strong>s which are TN earthed, the supply utilities usually protect the<br />
<strong>installation</strong> simply by means <strong>of</strong> sealed fuse cut-outs immediately upstream <strong>of</strong> the<br />
meter(s) (see Fig. P2). The consumer has no access to these fuses.<br />
Fuse … or …<br />
The incoming supply circuit-breaker (see Fig. P3)<br />
The consumer is allowed to operate this CB if necessary (e.g to reclose it if the<br />
current consumption has exceeded the authorized limit; to open it in case <strong>of</strong><br />
emergency or for isolation purposes).<br />
The rated residual current <strong>of</strong> the incoming circuit-breaker in the earth leakage<br />
protection shall be 300 mA.<br />
If If the <strong>installation</strong> is TT, the earth electrode resistance shall be less than<br />
50 V<br />
R<br />
300 mA 166<br />
� � � .. . In practice, the earth electrode resistance <strong>of</strong> a new <strong>installation</strong><br />
shall be less than 80 Ω Ω� Ω ( R<br />
2 )..<br />
The control and distribution board (consumer unit) (see Fig. P4)<br />
This board comprises:<br />
b A control panel for mounting (where appropriate) the incoming supply circuitbreaker<br />
and other control auxiliaries, as required<br />
b A distribution panel for housing 1, 2 or 3 rows (<strong>of</strong> 24 multi 9 units) or similar MCBs<br />
or fuse units, etc.<br />
b Installation accessories for fixing conductors, and rails for mounting MCBs, fuses<br />
bases, etc, neutral busbar and earthing bar, and so on<br />
b Service cable ducts or conduits, surface mounted or in cable chases embedded in<br />
the wall<br />
Note: to facilitate future modifications to the <strong>installation</strong>, it is recommended to keep<br />
all relevant documents (photos, diagrams, characteristics, etc.) in a suitable location<br />
close to the distribution board.<br />
The board should be installed at a height such that the operating handles,<br />
indicating dials (<strong>of</strong> meters) etc., are between 1 metre and 1.80 metres from the floor<br />
(1.30 metres in situations where handicapped or elderly people are concerned).<br />
Lightning arresters<br />
The <strong>installation</strong> <strong>of</strong> lightning arresters at the service position <strong>of</strong> a LV <strong>installation</strong> is<br />
strongly recommended for <strong>installation</strong>s which include sensitive (e.g electronic)<br />
equipment.<br />
These devices must automatically disconnect themselves from the <strong>installation</strong> in<br />
case <strong>of</strong> failure or be protected by a MCB. In the case <strong>of</strong> residential <strong>installation</strong>s, the<br />
use <strong>of</strong> a 300 mA differential incoming supply circuit-breaker type S (i.e slightly timedelayed)<br />
will provide effective earth leakage protection, while, at the same time, will<br />
not trip unnecessarily each time a lightning arrester discharges the current (<strong>of</strong> an<br />
overvoltage-surge) to earth.<br />
Resistance value <strong>of</strong> the earth electrode<br />
In the case where the resistance to earth exceeds 80 Ω, one or several 30 mA RCDs<br />
should be used in place <strong>of</strong> the earth leakage protection <strong>of</strong> the incoming supply<br />
circuit-breaker.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Meter<br />
Circuit breaker<br />
depending on<br />
earthing system<br />
Fig. P2 : Components <strong>of</strong> a control and distribution board<br />
Distribution<br />
board<br />
P3<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
P4<br />
P - Residential and other special locations<br />
Where utility power supply systems and<br />
consumers’ <strong>installation</strong>s form a TT earthed<br />
system, the governing standards impose the use<br />
<strong>of</strong> RCDs to ensure the protection <strong>of</strong> persons<br />
Residential and similar premises<br />
.3 Protection <strong>of</strong> people<br />
On TT earthed systems, the protection <strong>of</strong> persons is ensured by the following<br />
measures:<br />
b Protection against indirect contact hazards by RCDs (see Fig. P5) <strong>of</strong> medium<br />
sensitivity (300 mA) at the origin <strong>of</strong> the <strong>installation</strong> (incorporated in the incoming<br />
supply circuit-breaker or, on the incoming feed to the distribution board). This<br />
measure is associated with a consumer installed earth electrode to which must be<br />
connected the protective earth conductor (PE) from the exposed conductive parts <strong>of</strong><br />
all class I insulated appliances and equipment, as well as those from the earthing<br />
pins <strong>of</strong> all socket outlets<br />
b When the CB at the origin <strong>of</strong> an <strong>installation</strong> has no RCD protection, the protection<br />
<strong>of</strong> persons shall be ensured by class II level <strong>of</strong> insulation on all circuits upstream<br />
<strong>of</strong> the first RCDs. In the case where the distribution board is metallic, care shall be<br />
taken that all live parts are double insulated (supplementary clearances or insulation,<br />
use <strong>of</strong> covers, etc.) and wiring reliably fixed<br />
b Obligatory protection by 30 mA sensitive RCDs <strong>of</strong> socket outlet circuits, and<br />
circuits feeding bathroom, laundry rooms, and so on (for details <strong>of</strong> this latter<br />
obligation, refer to clause 3 <strong>of</strong> this chapter)<br />
Diverse<br />
circuits<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Socket-outlets<br />
circuit<br />
300 mA<br />
30 mA 30 mA<br />
Bathroom and/or<br />
shower room<br />
Fig. P5 : Installation with incoming-supply circuit-breaker having instantaneous differential<br />
protection<br />
Incoming supply circuit-breaker with instantaneous differential<br />
relay<br />
In this case:<br />
b An insulation fault to earth could result in a shutdown <strong>of</strong> the entire <strong>installation</strong><br />
b Where a lightning arrester is installed, its operation (i.e. discharging a voltage<br />
surge to earth) could appear to an RCD as an earth fault, with a consequent<br />
shutdown <strong>of</strong> the <strong>installation</strong><br />
Recommendation <strong>of</strong> suitable Merlin Gerin components<br />
b Incoming supply circuit-breaker with 300 mA differential and<br />
b High sensitivity 30 mA RCD (for example differential circuit-breaker 1P + N type<br />
Declic Vigi) on the circuits supplying socket outlets<br />
b High sensitivity 30 mA RCD (for example differential load switch type ID’clic) on<br />
circuits to bathrooms, shower rooms, laundry rooms, etc. (lighting, heating, socket<br />
outlets)<br />
Incoming supply circuit-breaker with type S time delayed<br />
differential relay<br />
This type <strong>of</strong> CB affords protection against fault to earth, but by virtue <strong>of</strong> a short time<br />
delay, provides a measure <strong>of</strong> discrimination with downstream instantaneous RCDs.<br />
Tripping <strong>of</strong> the incoming supply CB and its consequences (on deep freezers, for<br />
example) is thereby made less probable in the event <strong>of</strong> lightning, or other causes <strong>of</strong><br />
voltage surges. The discharge <strong>of</strong> voltage surge current to earth, through the surge<br />
arrester, will leave the type S circuit-breaker unaffected.
P - Residential and other special locations<br />
1<br />
Diverse<br />
circuits<br />
2<br />
5 3 4<br />
300 mA 30 mA 30 mA 30 mA<br />
High-risk circuit<br />
(dish-washing<br />
machine)<br />
Socket-outlet<br />
circuit<br />
Bathroom and/or<br />
shower room<br />
Fig. P7 : Installation with incoming-supply circuit-breaker<br />
having no differential protection<br />
Residential and similar premises<br />
Recommendation <strong>of</strong> suitable Merlin Gerin components (see Fig. P6)<br />
b Incoming supply circuit-breaker with 300 mA differential type S and<br />
b High sensitivity 30 mA RCD (for example differential circuit-breaker 1P + N type<br />
Declic Vigi) on the circuits supplying washing machines and dish-washing machine<br />
b High sensitivity 30 mA RCD (for example differential load switch type ID’clic) on<br />
circuits to bathrooms, shower rooms, laundry rooms, etc. (lighting, heating, socket<br />
outlets)<br />
Diverse<br />
circuits<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
High-risk location<br />
(laundry room)<br />
300 mA - type S<br />
30 mA 30 mA 30 mA<br />
Socketoutlet<br />
circuit<br />
Bathroom and/or<br />
shower room<br />
Fig. P6 : Installation with incoming-supply circuit-breaker having short time delay differential<br />
protection, type S<br />
Incoming supply circuit-breaker without differential protection<br />
In this case the protection <strong>of</strong> persons must be ensured by:<br />
b Class II level <strong>of</strong> insulation up to the downstream terminals <strong>of</strong> the RCDs<br />
b All outgoing circuits from the distribution board must be protected by 30 mA or<br />
300 mA RCDs according to the type <strong>of</strong> circuit concerned as discussed in chapter F.<br />
Where a voltage surge arrester is installed upstream <strong>of</strong> the distribution board<br />
(to protect sensitive electronic equipment such as microprocessors, videocassette<br />
recorders, TV sets, electronic cash registers, etc.) it is imperative that the<br />
device automatically disconnects itself from the <strong>installation</strong> following a rare (but<br />
always possible) failure. Some devices employ replaceable fusing elements; the<br />
recommended method however as shown in Figure P7, is to use a circuit-breaker.<br />
Recommendation <strong>of</strong> suitable Merlin Gerin components<br />
Figure P7 refers:<br />
1. Incoming-supply circuit-breaker without differential protection<br />
2. Automatic disconnection device (if a lightning arrester is installed)<br />
3. 30 mA RCD (for example differential circuit-breaker 1P + N type Declic Vigi) on<br />
each circuit supplying one or more socket-outlets<br />
4. 30 mA RCD (for example differential load swith type ID’clic) on circuits to<br />
bathrooms and shower rooms (lighting, heating and socket-outlets) or a 30 mA<br />
differential circuit-breaker per circuit<br />
5. 300 mA RCD (for example differential load swith) on all the other circuits<br />
P5<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
P6<br />
P - Residential and other special locations<br />
The distribution and division <strong>of</strong> circuits provides<br />
comfort and facilitates rapid location <strong>of</strong> fault<br />
Socketoutlets<br />
Lighting Heating<br />
Washing<br />
machine<br />
Cooking<br />
apparatus<br />
.4 Circuits<br />
Subdivision<br />
National standards commonly recommend the subdivision <strong>of</strong> circuits according to the<br />
number <strong>of</strong> utilization categories in the <strong>installation</strong> concerned (see Fig. P8):<br />
b At least 1 circuit for lighting. Each circuit supplying a maximum <strong>of</strong> 8 lighting points<br />
b At least 1 circuit for socket-outlets rated 10/16 A, each circuit supplying a maximum<br />
<strong>of</strong> 8 sockets. These sockets may be single or double units (a double unit is made up<br />
<strong>of</strong> two 10/16 A sockets mounted on a common base in an embedded box, identical<br />
to that <strong>of</strong> a single unit<br />
b 1 circuit for each appliance such as water heater, washing machine, dish-washing<br />
machine, cooker, refrigerator, etc. Recommended numbers <strong>of</strong> 10/16 A (or similar)<br />
socket-outlets and fixed lighting points, according to the use for which the various<br />
rooms <strong>of</strong> a dwelling are intended, are indicated in Figure P9<br />
Fig. P8 : Circuit division according to utilization Fig P9 : Recommended minimum number <strong>of</strong> lighting and power points in residential premises<br />
The inclusion <strong>of</strong> a protective conductor in all<br />
circuits is required by IEC and most national<br />
standards<br />
Residential and similar premises<br />
Room function Minimum number Minimum number<br />
<strong>of</strong> fixed lighting points <strong>of</strong> 0/ 6 A socket-outlets<br />
Living room 1 5<br />
Bedroom, lounge, 1 3<br />
bureau, dining room<br />
Kitchen 2 4 (1)<br />
Bathroom, shower room 2 1 or 2<br />
Entrance hall, box room 1 1<br />
WC, storage space 1 -<br />
Laundry room - 1<br />
(1) Of which 2 above the working surface and 1 for a specialized circuit: in addition<br />
an independent socket-outlet <strong>of</strong> 16 A or 20 A for a cooker and a junction box or<br />
socket-outlet for a 32 A specialized circuit<br />
Protective conductors<br />
IEC and most national standards require that each circuit includes a protective<br />
conductor. This practice is strongly recommended where class I insulated appliances<br />
and equipment are installed, which is the general case.<br />
The protective conductors must connect the earthing-pin contact in each socketoutlet,<br />
and the earthing terminal in class I equipment, to the main earthing terminal<br />
at the origin <strong>of</strong> the <strong>installation</strong>.<br />
Furthermore, 10/16 A (or similarly sized) socket-outlets must be provided with<br />
shuttered contact orifices.<br />
Cross-sectional-area (c.s.a.) <strong>of</strong> conductors (see Fig. P 0)<br />
The c.s.a. <strong>of</strong> conductors and the rated current <strong>of</strong> the associated protective device<br />
depend on the current magnitude <strong>of</strong> the circuit, the ambient temperature, the kind <strong>of</strong><br />
<strong>installation</strong>, and the influence <strong>of</strong> neighbouring circuits (refer to chapter G)<br />
Moreover, the conductors for the phase wires, the neutral and the protective<br />
conductors <strong>of</strong> a given circuit must all be <strong>of</strong> equal c.s.a. (assuming the same material<br />
for the conductors concerned, i.e. all copper or all aluminium).<br />
Fig. P10 : Circuit-breaker 1 phase + N - 2 x 9 mm spaces<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
P - Residential and other special locations<br />
Residential and similar premises<br />
Figure P indicates the c.s.a. required for commonly-used appliances<br />
Protective devices 1 phase + N in 2 x 9 mm spaces comply with requirements for<br />
isolation, and for marking <strong>of</strong> circuit current rating and conductor sizes.<br />
Type <strong>of</strong> circuit c. s. a. <strong>of</strong> the Maximum power Protective device<br />
single-phase 230 V conductors<br />
ph + N or ph + N + PE<br />
Fixed lighting 1.5 mm 2 2,300 W Circuit-breaker 16 A<br />
(2.5 mm 2 ) Fuse 10 A<br />
10/16 A 2.5 mm 2 4,600 W Circuit-breaker 25 A<br />
(4 mm 2 ) Fuse 20 A<br />
Individual-load circuits<br />
Water heater 2.5 mm 2 4,600 W Circuit-breaker 25 A<br />
(4 mm 2 ) Fuse 20 A<br />
Dish-washing machine 2.5 mm 2 4,600 W Circuit-breaker 25 A<br />
(4 mm 2 ) Fuse 20 A<br />
Clothes-washing machine 2.5 mm 2 4,600 W Circuit-breaker 25 A<br />
(4 mm 2 ) Fuse 20 A<br />
Cooker or hotplate (1) 6 mm 2 7,300 W Circuit-breaker 40 A<br />
(10 mm 2 ) Fuse 32 A<br />
Electric space heater 1.5 mm 2 2,300 W Circuit-breaker 16 A<br />
(2.5 mm 2 ) Fuse 10 A<br />
(1) In a 230/400 V 3-phase circuit, the c. s. a. is 4 mm 2 for copper or 6 mm 2 for aluminium, and protection is provided by a 32 A<br />
circuit-breaker or by 25 A fuses.<br />
Fig. P11 : C. s. a. <strong>of</strong> conductors and current rating <strong>of</strong> the protective devices in residential <strong>installation</strong>s (the c. s. a. <strong>of</strong> aluminium conductors are shown in brackets)<br />
.5 Protection against overvoltages and lightning<br />
The choice <strong>of</strong> surge arrester is described in chapter J<br />
Installation <strong>rules</strong><br />
Three principal <strong>rules</strong> must be respected:<br />
1 - It is imperative that the three lengths <strong>of</strong> cable used for the <strong>installation</strong> <strong>of</strong> the surge<br />
arrester each be less than 50 cm i.e.:<br />
b the live conductors connected to the isolating switch<br />
b from the isolating switch to the surge arrester<br />
b from the surge arrester to the main distribution board (MDB) earth bar (not<br />
to be confused with the main protective-earth (PE) conductor or the main earth<br />
terminal for the <strong>installation</strong>.The MDB earth bar must evidently be located in the<br />
same cabinet as the surge arrester.<br />
2 - It is necessary to use an isolating switch <strong>of</strong> a type recommended by the<br />
manufacturer <strong>of</strong> the surge arrester.<br />
3 - In the interest <strong>of</strong> a good continuity <strong>of</strong> supply it is recommended that the<br />
circuit-breaker be <strong>of</strong> the time-delayed or selective type.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
P7<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
P<br />
P - Residential and other special locations<br />
2 Bathrooms and showers<br />
Bathrooms and showers rooms are areas <strong>of</strong> high risk, because <strong>of</strong> the very low<br />
resistance <strong>of</strong> the human body when wet or immersed in water.<br />
Precaution to be taken are therefore correspondingly rigorous, and the regulations<br />
are more severe than those for most other locations.<br />
The relevant standard is IEC 60364-7-701.<br />
Precautions to observe are based on three aspects:<br />
b The definition <strong>of</strong> zones, numbered 0,1, 2, 3 in which the placement (or exclusion)<br />
<strong>of</strong> any <strong>electrical</strong> device is strictly limited or forbidden and, where permitted, the<br />
<strong>electrical</strong> and mechanical protection is prescribed<br />
b The establishment <strong>of</strong> an equipotential bond between all exposed and extraneous<br />
metal parts in the zones concerned<br />
b The strict adherence to the requirements prescribed for each particular zones, as<br />
tabled in clause 3<br />
2.1 Classification <strong>of</strong> zones<br />
Sub-clause 701.32 <strong>of</strong> IEC 60364-7-701 defines the zones 0, 1, 2, 3 as shown in the<br />
following diagrams (see Fig. P12 below to Fig P1 opposite and next pages):<br />
Zone 1*<br />
Zone 0<br />
Zone 1<br />
Zone 1<br />
Zone 0<br />
Zone 2 Zone 3<br />
0.60 m 2.40 m<br />
Zone 2<br />
0.60 m<br />
(*) Zone 1 is above the bath as shown in the vertical cross-section<br />
Fig. P12 : Zones 0, 1, 2 and 3 in proximity to a bath-tub<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Zone 1*<br />
Zone 0<br />
Zone 3<br />
2.40 m<br />
0.60 m<br />
Zone 2 Zone 3<br />
2.25 m<br />
2.40 m
P - Residential and other special locations<br />
2 Bathrooms and showers<br />
Zone 0<br />
Zone 1<br />
0.60 m<br />
Zone 3<br />
2.40 m<br />
Zone 1 Zone 2<br />
Zone 3<br />
Zone 1<br />
Zone 0<br />
Zone 2<br />
0.60 m<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
2.40 m<br />
Zone 0<br />
Zone 1<br />
0.60 m<br />
2.25 m<br />
Fig. P13 : Zones 0, 1, 2 and 3 in proximity <strong>of</strong> a shower with basin<br />
Zone 1<br />
Zone 2<br />
Zone 3<br />
Zone 1<br />
0.60 m<br />
Fixed shower<br />
head (1)<br />
0.60 m<br />
Zone<br />
2<br />
2.40 m<br />
Zone 3<br />
Zone 2<br />
Fixed shower<br />
head (1)<br />
0.60 m<br />
Zone 1<br />
0.60 m<br />
Zone 2<br />
Zone 3<br />
2.25 m<br />
2.40 m<br />
Zone 3<br />
2.40 m<br />
(1) When the shower head is at the end <strong>of</strong> a flexible tube, the vertical central axis <strong>of</strong><br />
a zone passes through the fixed end <strong>of</strong> the flexible tube<br />
Fig. P14 : Zones 0, 1, 2 and 3 in proximity <strong>of</strong> a shower without basin<br />
Prefabricated<br />
shower<br />
cabinet<br />
0.60 m<br />
0.60 m<br />
Fig. P15 : No switch or socket-outlet is permitted within 60 cm <strong>of</strong> the door opening <strong>of</strong> a shower<br />
cabinet<br />
P<br />
© Schneider Electric - all rights reserved
P10<br />
© Schneider Electric - all rights reserved<br />
P - Residential and other special locations<br />
Classes<br />
<strong>of</strong> external<br />
influences<br />
AD 3<br />
BB 2<br />
BC 3<br />
AD 3<br />
BB 3<br />
BC 3<br />
AD 7<br />
BB 3<br />
BC 3<br />
Fig. P16 : Individual showers with dressing cubicles<br />
Classes<br />
<strong>of</strong> external<br />
influences<br />
h < 1.10m<br />
AD 5<br />
1.10m < h < 2.25m<br />
AD 3<br />
BB 3<br />
BC 3<br />
AD 7<br />
BB 3<br />
BC 3<br />
Zone 3<br />
Dressing cubicles (zone 2)<br />
Shower cabinets (zone 1)<br />
Dressing cubicles<br />
Zone 1<br />
Fig. P17 : Individual showers with separate individual dressing cubicles<br />
Classes<br />
<strong>of</strong> external<br />
influences<br />
AD 3<br />
BB 2<br />
BC 3<br />
h < 1.10m<br />
AD 5<br />
1.10m < h < 2.25m<br />
AD 3<br />
BB 3<br />
BC 3<br />
Fig. P18 : Communal showers and common dressing room<br />
Zone 2<br />
2 Bathrooms and showers<br />
Zone 2<br />
Dressing room<br />
Zone 2<br />
Zone 1<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Classes<br />
<strong>of</strong> external<br />
influences<br />
AD 3<br />
BB 2<br />
BC 3<br />
WC<br />
WC<br />
AD 3<br />
BB 2<br />
BC 3<br />
Classes<br />
<strong>of</strong> external<br />
influences<br />
h < 1.10m<br />
AD 5<br />
1.10m < h < 2.25m<br />
AD 3<br />
BB 3<br />
BC 3<br />
Classes<br />
<strong>of</strong> external<br />
influences<br />
AD 7<br />
BB 3<br />
BC 3<br />
Note: Classes <strong>of</strong> external influences (see Fig. E46).<br />
AD 3<br />
BB 2<br />
BC 3<br />
h < 1.10m<br />
AD 5<br />
1.10m < h < 2.25m<br />
AD 3<br />
BB 3<br />
BC 3
P - Residential and other special locations<br />
2 Bathrooms and showers<br />
2.2 Equipotential bonding (see Fig. P1 )<br />
Water-drainage<br />
piping<br />
Socket-outlet<br />
Lighting<br />
Gaz<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
To the earth<br />
electrode<br />
Metal bath Equipotential conductors<br />
for a bathroom<br />
Fig. P19 : Supplementary equipotential bonding in a bathroom<br />
Metal<br />
door-frame<br />
2.3 Requirements prescribed for each zone<br />
Metallic pipes<br />
h i 2 m<br />
Radiator<br />
The table <strong>of</strong> clause 3 describes the application <strong>of</strong> the principles mentioned in the<br />
foregoing text and in other similar or related cases<br />
P11<br />
© Schneider Electric - all rights reserved
P12<br />
© Schneider Electric - all rights reserved<br />
P - Residential and other special locations<br />
3 Recommendations applicable to<br />
special <strong>installation</strong>s and locations<br />
Figure P20 below summarizes the main requirements prescribed in many national<br />
and international standards.<br />
Note: Section in brackets refer to sections <strong>of</strong> IEC 60364-7<br />
Locations Protection principles IP Wiring Switchgear Socket-outlets Installation<br />
level and cables materials<br />
Domestic dwellings b TT or TN-S systems 20 Switch operating handles Protection by<br />
and other habitations b Differential protection and similar devices on 30 mA RCDs<br />
v 300 mA if the earth electrode distribution panels,<br />
resistance is y 80 ohms instantaneous to be mounted<br />
or short time delay (type S) between 1 metre and<br />
v 30 mA if the earth electrode 1.80 metre above the floor<br />
resistance is u 500 ohms<br />
b surge arrester at the origin <strong>of</strong> the<br />
<strong>installation</strong> if<br />
v supply is from overhead line with bare<br />
conductors, and if<br />
v the keraunic level > 25<br />
b a protective earth (PE) conductor<br />
on all circuits<br />
Bathrooms or shower Supplementary equipotential bonding<br />
rooms (section 701) in zones 0, 1, 2 and 3<br />
Zone 0 SELV 12 V only 27 Class II Special appliances<br />
limited to<br />
strict minimum<br />
Zone 1 SELV 12 V 25 Class II Special aplliances<br />
limited to Water heater<br />
strict minimum<br />
Zone 2 SELV 12 V or 30 mA RCD 24 Class II Special appliances<br />
limited to Water heater<br />
strict minimum Class II luminaires<br />
Zone 3 21 Only socket-outlets protected by :<br />
b 30 mA RCD or<br />
b Electrical separation or<br />
b SELV 50 V<br />
Swimming baths Supplementary equipotential bonding<br />
(section 702) in zones 0, 1, and 2<br />
Zone 0 SELV 12 V 28 Class II Special appliances<br />
limited to<br />
strict minimum<br />
Zone 1 25 Class II Special appliances<br />
limited to<br />
strict minimum<br />
Zone 2 22 Only socket-outlets protected by :<br />
(indoor) b 30 mA RCD or<br />
24 b <strong>electrical</strong> separation or<br />
(outdoor) b SELV 50 V<br />
Saunas 24 Class II Adapted to temperature<br />
(section 703)<br />
Work sites Conventional voltage limit UL 44 Mechanically Protection by<br />
(section 704) reduced to 25 V protected 30 mA RCDs<br />
Agricultural and Conventional voltage limit UL 35 Protection by<br />
horticultural reduced to 25 V 30 mA RCDs<br />
establishments Protection against fire risks<br />
(section 705) by 500 mA RCDs<br />
Restricted conductive 2x Protection <strong>of</strong>:<br />
locations (section 706) b Portable tools by:<br />
v SELV or<br />
v Electrical separation<br />
b Hand-held lamps<br />
v By SELV<br />
b Fixed equipement by<br />
v SELV<br />
v Electrical separation<br />
v 30 mA RCDs<br />
v Special supplementary<br />
equipotential bonding<br />
Fig. P20 : Main requirements prescribed in many national and international standards (continued on opposite page)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008
P - Residential and other special locations<br />
3 Recommendations applicable to<br />
special <strong>installation</strong>s and locations<br />
Locations Protection principles IP Wiring Switchgear Socket-outlets Installation<br />
level and cables materials<br />
Fountains Protection by 30 mA RCDs and<br />
(section 702) equipotential bonding <strong>of</strong> all exposed<br />
and extraneous conductive parts<br />
Data processing TN-S system recommended<br />
(section 707) TT system if leakage current is limited.<br />
Protective conductor 10 mm 2 minimum<br />
in aluminium. Smaller sizes (in copper)<br />
must be doubled.<br />
Caravan park 55 Flexible cable <strong>of</strong> Socket-outlets<br />
(section 708) 25 metres shall be placed<br />
length at a height <strong>of</strong><br />
0.80 m to 1.50 m<br />
from the ground.<br />
Protection <strong>of</strong><br />
circuits by<br />
30 mA RCDs<br />
(one per 6<br />
socket-outlets)<br />
Marinas and pleasure The cable length for connection to Protection <strong>of</strong><br />
craft (section 709) pleasure craft must not exceeded 25 m circuits by<br />
30 mA RCDs<br />
(one per 6<br />
socket-outlets)<br />
Medical locations IT medical system equipotential bonding Protection by<br />
(section 710) 30 mA RCDs<br />
Exhibitions, shows and TT or TN-S systems 4x Protection by<br />
stands (section 711) 30 mA RCDs<br />
Balneotherapy Individual: see section 701<br />
(cure-centre baths) (volumes 0 and 1)<br />
Collective: see section 702<br />
(volumes 0 and 1)<br />
Motor-fuel filling Explosion risks in security zones Limited to the<br />
stations necessary minimum<br />
Motor vehicules Protection by RCDs or by<br />
<strong>electrical</strong> separation<br />
External lighting 23 Protection by<br />
<strong>installation</strong>s 30 mA RCDs<br />
(section 714)<br />
Mobile or transportable The use <strong>of</strong> TN-C system is not 30 mA RCDs<br />
units (section 717) permitted inside any unit must be used for<br />
all socket-outlets<br />
supplying<br />
equipment<br />
outside the unit<br />
Fig. P20 : Main requirements prescribed in many national and international standards (concluded)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
P13<br />
© Schneider Electric - all rights reserved
2<br />
3<br />
4<br />
5<br />
<strong>Chapter</strong> Q<br />
EMC guidelines<br />
Contents<br />
Electrical distribution Q2<br />
Earthing principles and structures Q3<br />
Implementation Q5<br />
3.1 Equipotential bonding inside and outside buildings Q5<br />
3.2 Improving equipotential conditions Q5<br />
3.3 Separating cables Q7<br />
3.4 False floor Q7<br />
3.5 Cable running Q8<br />
3.6 Implementation <strong>of</strong> shielded cables Q11<br />
3.7 Communication networks Q11<br />
3.8 Implementation <strong>of</strong> surge arrestors Q12<br />
3.9 Cabinet cabling Q13<br />
3.10 Standards Q13<br />
Coupling mechanisms and counter-measures Q 4<br />
4.1 <strong>General</strong> Q14<br />
4.2 Common-mode impedance coupling Q15<br />
4.3 Capacitive coupling Q16<br />
4.4 Inductive coupling Q17<br />
4.5 Radiated coupling Q18<br />
Wiring recommendations Q20<br />
5.1 Signal classes Q20<br />
5.2 Wiring recommendations Q20<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Q<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
Q2<br />
Q - EMC guidelines<br />
Fig. Q1 : Main characteristics <strong>of</strong> system earthing<br />
Electrical distribution<br />
The system earthing arrangement must be properly selected to ensure the<br />
safety <strong>of</strong> life and property. The behaviour <strong>of</strong> the different systems with respect<br />
to EMC considerations must be taken into account. Figure Q below presents a<br />
summary <strong>of</strong> their main characteristics.<br />
European standards (see EN 50174-2 § 6.4 and EN 50310 § 6.3) recommend the<br />
TN-S system which causes the fewest EMC problems for <strong>installation</strong>s comprising<br />
information-technology equipment (including telecom equipment).<br />
TT TN-S IT TN-C<br />
Safety <strong>of</strong> persons Good Good<br />
RCD mandatory Continuity <strong>of</strong> the PE conductor must be ensured throughout the <strong>installation</strong><br />
Safety <strong>of</strong> property Good Poor Good Poor<br />
Medium fault current High fault current Low current for first fault High fault current<br />
(< a few dozen amperes) (around 1 kA) (< a few dozen mA), (around 1 kA)<br />
but high for second fault<br />
Availability <strong>of</strong> energy Good Good Excellent Good<br />
EMC behaviour Good Excellent Poor (to be avoided) Poor<br />
- Risk <strong>of</strong> overvoltages - Few equipotential - Risk <strong>of</strong> overvoltages (should never be used)<br />
- Equipotential problems - Common-mode filters - Neutral and PE are<br />
problems - Need to manage and surge arrestors the same<br />
- Need to manage devices with high must handle the phase- - Circulation <strong>of</strong> disturbed<br />
devices with high leakage currents to-phase voltages currents in exposed<br />
leakage currents - High fault currents - RCDs subject to conductive parts (high<br />
(transient disturbances) nuisance tripping if magnetic-field radiation)<br />
common-mode - High fault currents<br />
capacitors are present (transient disturbances)<br />
- Equivalent to<br />
TN system for second<br />
fault<br />
When an <strong>installation</strong> includes high-power equipment (motors, air-conditioning, lifts,<br />
power electronics, etc.), it is advised to install one or more transformers specifically<br />
for these systems. Electrical distribution must be organised in a star system and all<br />
outgoing circuits must exit the main low-voltage switchboard (MLVS).<br />
Electronic systems (control/monitoring, regulation, measurement instruments, etc.)<br />
must be supplied by a dedicated transformer in a TN-S system.<br />
Figure Q2 below illustrate these recommendations.<br />
Disturbing<br />
devices<br />
Transformer<br />
Sensitive<br />
devices<br />
Disturbing<br />
devices<br />
Not recommended Preferable Excellent<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Sensitive<br />
devices<br />
Fig. Q2 : Recommendations <strong>of</strong> separated distributions<br />
Lighting<br />
Air conditioning<br />
Disturbing<br />
devices<br />
Sensitive<br />
devices
Q - EMC guidelines<br />
2 Earthing principles and<br />
structures<br />
This section deals with the earthing and equipotential bonding <strong>of</strong> information-technology<br />
devices and other similar devices requiring interconnections for signalling purposes.<br />
Earthing networks are <strong>design</strong>ed to fulfil a number <strong>of</strong> functions. They can be<br />
independent or operate together to provide one or more <strong>of</strong> the following:<br />
b Safety <strong>of</strong> persons with respect to <strong>electrical</strong> hazards<br />
b Protection <strong>of</strong> equipment with respect to <strong>electrical</strong> hazards<br />
b A reference value for reliable, high-quality signals<br />
b Satisfactory EMC performance<br />
The system earthing arrangement is generally <strong>design</strong>ed and installed in view <strong>of</strong><br />
obtaining a low impedance capable <strong>of</strong> diverting fault currents and HF currents away<br />
from electronic devices and systems. There are different types <strong>of</strong> system earthing<br />
arrangements and some require that specific conditions be met. These conditions<br />
are not always met in typical <strong>installation</strong>s. The recommendations presented in this<br />
section are intended for such <strong>installation</strong>s.<br />
For pr<strong>of</strong>essional and industrial <strong>installation</strong>s, a common bonding network (CBN) may<br />
be useful to ensure better EMC performance with respect to the following points:<br />
b Digital systems and new technologies<br />
b Compliance with the EMC requirements <strong>of</strong> EEC 89/336 (emission and immunity)<br />
b The wide number <strong>of</strong> <strong>electrical</strong> applications<br />
b A high level <strong>of</strong> system safety and security, as well as reliability and/or availability<br />
For residential premises, however, where the use <strong>of</strong> <strong>electrical</strong> devices is limited, an<br />
isolated bonding network (IBN) or, even better, a mesh IBN may be a solution.<br />
It is now recognised that independent, dedicated earth electrodes, each serving a<br />
separate earthing network, are a solution that is not acceptable in terms <strong>of</strong> EMC,<br />
but also represent a serious safety hazard. In certain countries, the national building<br />
codes forbid such systems.<br />
Use <strong>of</strong> a separate “clean” earthing network for electronics and a “dirty” earthing<br />
network for energy is not recommended in view <strong>of</strong> obtaining correct EMC, even<br />
when a single electrode is used (see Fig. Q and Fig. Q4). In the event <strong>of</strong> a lightning<br />
strike, a fault current or HF disturbances as well as transient currents will flow in the<br />
<strong>installation</strong>. Consequently, transient voltages will be created and result in failures or<br />
damage to the <strong>installation</strong>. If <strong>installation</strong> and maintenance are carried out properly,<br />
this approach may be dependable (at power frequencies), but it is generally not<br />
suitable for EMC purposes and is not recommended for general use.<br />
"Clean"<br />
earthing network<br />
Fig. Q3 : Independent earth electrodes, a solution generally not acceptable for safety and EMC<br />
reasons<br />
"Clean"<br />
earthing network<br />
Fig. Q4 : Installation with a single earth electrode<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Electrical<br />
earthing network<br />
Separate earth electrodes<br />
Electrical<br />
earthing network<br />
Single earth electrode<br />
Surge arrestors<br />
Surge arrestors<br />
Q<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved Q4<br />
Q - EMC guidelines<br />
Fig. Q6 : Each level has a mesh and the meshes are<br />
interconnected at several points between levels. Certain<br />
ground-floor meshes are reinforced to meet the needs <strong>of</strong><br />
certain areas<br />
2 Earthing principles and<br />
structures<br />
The recommended configuration for the earthing network and electrodes is two or<br />
three dimensional (see Fig. Q5). This approach is advised for general use, both<br />
in terms <strong>of</strong> safety and EMC. This recommendation does not exclude other special<br />
configurations that, when correctly maintained, are also suitable.<br />
Fig. Q5 : Installation with multiple earth electrodes<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Equipotential bonding required for<br />
multi-level buildings<br />
"Electrical" and "communication"<br />
earthing as needed<br />
Surge arrestors<br />
Multiple interconnected earth electrodes<br />
In a typical <strong>installation</strong> for a multi-level building, each level should have its<br />
own earthing network (generally a mesh) and all the networks must be both<br />
interconnected and connected to the earth electrode. At least two connections are<br />
required (built in redundancy) to ensure that, if one conductor breaks, no section <strong>of</strong><br />
the earthing network is isolated.<br />
Practically speaking, more than two connections are made to obtain better symmetry<br />
in current flow, thus reducing differences in voltage and the overall impedance<br />
between the various levels in the building.<br />
The many parallel paths have different resonance frequencies. If one path has a high<br />
impedance, it is most probably shunted by another path with a different resonance<br />
frequency. On the whole, over a wide frequency spectrum (dozens <strong>of</strong> Hz and MHz), a<br />
large number <strong>of</strong> paths results in a low-impedance system (see Fig. Q6).<br />
Each room in the building should have earthing-network conductors for equipotential<br />
bonding <strong>of</strong> devices and systems, cableways, trunking systems and structures. This<br />
system can be reinforced by connecting metal pipes, gutters, supports, frames, etc.<br />
In certain special cases, such as control rooms or computers installed on false floors,<br />
ground reference plane or earthing strips in areas for electronic systems can be used<br />
to improve earthing <strong>of</strong> sensitive devices and protection interconnection cables.
Q - EMC guidelines<br />
3 Implementation<br />
3.1 Equipotential bonding inside and outside<br />
buildings<br />
The fundamental goals <strong>of</strong> earthing and bonding are the following:<br />
b Safety<br />
By limiting the touch voltage and the return path <strong>of</strong> fault currents<br />
b EMC<br />
By avoiding differences in potential and providing a screening effect.<br />
Stray currents are inevitably propagated in an earthing network. It is impossible to<br />
eliminate all the sources <strong>of</strong> disturbances for a site. Earth loops are also inevitable.<br />
When a magnetic field affects a site, e.g. the field created by lightning, differences in<br />
potential are created in the loops formed by the various conductors and the currents<br />
flowing in the earthing system. Consequently, the earthing network is directly<br />
affected by any counter-measures taken outside the building.<br />
As long as the currents flow in the earthing system and not in the electronic circuits,<br />
they do no damage. However, when earthing networks are not equipotential, e.g.<br />
when they are star connected to the earth electrode, the HF stray currents will flow<br />
wherever they can, including in control wires. Equipment can be disturbed, damaged<br />
or even destroyed.<br />
The only inexpensive means to divide the currents in an earthing system and<br />
maintain satisfactory equipotential characteristics is to interconnect the earthing<br />
networks. This contributes to better equipotential bonding within the earthing<br />
system, but does not remove the need for protective conductors. To meet legal<br />
requirements in terms <strong>of</strong> the safety <strong>of</strong> persons, sufficiently sized and identified<br />
protective conductors must remain in place between each piece <strong>of</strong> equipment and<br />
the earthing terminal. What is more, with the possible exception <strong>of</strong> a building with a<br />
steel structure, a large number <strong>of</strong> conductors for the surge-arrestor or the lightningprotection<br />
network must be directly connected to the earth electrode.<br />
The fundamental difference between a protective conductor (PE) and a surgearrestor<br />
down-lead is that the first conducts internal currents to the neutral <strong>of</strong> the<br />
MV/LV transformer whereas the second carries external current (from outside the<br />
<strong>installation</strong>) to the earth electrode.<br />
In a building, it is advised to connect an earthing network to all accessible conducting<br />
structures, namely metal beams and door frames, pipes, etc. It is generally sufficient<br />
to connect metal trunking, cable trays and lintels, pipes, ventilation ducts, etc. at<br />
as many points as possible. In places where there is a large amount <strong>of</strong> equipment<br />
and the size <strong>of</strong> the mesh in the bonding network is greater than four metres, an<br />
equipotential conductor should be added. The size and type <strong>of</strong> conductor are not <strong>of</strong><br />
critical importance.<br />
It is imperative to interconnect the earthing networks <strong>of</strong> buildings that have shared<br />
cable connections. Interconnection <strong>of</strong> the earthing networks must take place via a<br />
number <strong>of</strong> conductors and all the internal metal structures <strong>of</strong> the buildings or linking<br />
the buildings (on the condition that they are not interrupted).<br />
In a given building, the various earthing networks (electronics, computing, telecom,<br />
etc.) must be interconnected to form a single equipotential bonding network.<br />
This earthing-network must be as meshed as possible. If the earthing network is<br />
equipotential, the differences in potential between communicating devices will be low<br />
and a large number <strong>of</strong> EMC problems disappear. Differences in potential are also<br />
reduced in the event <strong>of</strong> insulation faults or lightning strikes.<br />
If equipotential conditions between buildings cannot be achieved or if the distance<br />
between buildings is greater than ten metres, it is highly recommended to use<br />
optical fibre for communication links and galvanic insulators for measurement and<br />
communication systems.<br />
These measures are mandatory if the <strong>electrical</strong> supply system uses the IT or<br />
TN-C system.<br />
3.2 Improving equipotential conditions<br />
Bonding networks<br />
Even though the ideal bonding network would be made <strong>of</strong> sheet metal or a fine<br />
mesh, experience has shown that for most disturbances, a three-metre mesh size is<br />
sufficient to create a mesh bonding network.<br />
Examples <strong>of</strong> different bonding networks are shown in Figure Q7 next page. The<br />
minimum recommended structure comprises a conductor (e.g. copper cable or strip)<br />
surrounding the room.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Q<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
Q<br />
Q - EMC guidelines<br />
Trunk<br />
BN: Bonding network<br />
CBN: Common bonding network<br />
IBN: Isolated bonding network<br />
Fig. Q7 : Examples <strong>of</strong> bonding networks<br />
IBN<br />
Mesh IBN<br />
Tree structure<br />
Star (IBN)<br />
3 Implementation<br />
PE<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
IBN<br />
CBN<br />
Mesh BN<br />
Mesh BN<br />
Local mesh Local mesh<br />
IBN<br />
The length <strong>of</strong> connections between a structural element and the bonding network<br />
does not exceed 50 centimetres and an additional connection should be installed<br />
in parallel at a certain distance from the first. The inductance <strong>of</strong> the connection<br />
between the earthing bar <strong>of</strong> the <strong>electrical</strong> enclosure for a set <strong>of</strong> equipment and the<br />
bonding network (see below) should be less than one µHenry (0.5 µH, if possible).<br />
For example, it is possible to use a single 50 cm conductor or two parallel conductors<br />
one meter long, installed at a minimum distance from one another (at least 50 cm) to<br />
reduce the mutual inductance between the two conductors.<br />
Where possible, connection to the bonding network should be at an intersection to<br />
divide the HF currents by four without lengthening the connection. The pr<strong>of</strong>ile <strong>of</strong> the<br />
bonding conductors is not important, but a flat pr<strong>of</strong>ile is preferable. The conductor<br />
should also be as short as possible.<br />
Parallel earthing conductor (PEC)<br />
The purpose <strong>of</strong> a parallel earthing conductor is to reduce the common-mode current<br />
flowing in the conductors that also carry the differential-mode signal (the commonmode<br />
impedance and the surface area <strong>of</strong> the loop are reduced).<br />
The parallel earthing conductor must be <strong>design</strong>ed to handle high currents when it<br />
is used for protection against lightning or for the return <strong>of</strong> high fault currents. When<br />
cable shielding is used as a parallel earthing conductor, it cannot handle such high<br />
currents and the solution is to run the cable along metal structural elements or<br />
cableways which then act as other parallel earthing conductors for the entire cable.<br />
Another possibility is to run the shielded cable next to a large parallel earthing<br />
conductor with both the shielded cable and the parallel earthing conductor connected<br />
at each end to the local earthing terminal <strong>of</strong> the equipment or the device.<br />
For very long distances, additional connections to the network are advised for<br />
the parallel earthing conductor, at irregular distances between the devices. These<br />
additional connections form a shorter return path for the disturbing currents flowing<br />
through the parallel earthing conductor. For U-shaped trays, shielding and tubes, the<br />
additional connections should be external to maintain the separation with the interior<br />
(“screening” effect).<br />
Bonding conductors<br />
Bonding conductors may be metal strips, flat braids or round conductors. For highfrequency<br />
systems, metal strips and flat braids are preferable (skin effect) because a<br />
round conductor has a higher impedance than a flat conductor with the same cross<br />
section. Where possible, the length to width ratio should not exceed 5.
Q - EMC guidelines<br />
3 Implementation<br />
3.3 Separating cables<br />
The physical separation <strong>of</strong> high and low-current cables is very important for EMC,<br />
particularly if low-current cables are not shielded or the shielding is not connected<br />
to the exposed conductive parts (ECPs). The sensitivity <strong>of</strong> electronic equipment is in<br />
large part determined by the accompanying cable system.<br />
If there is no separation (different types <strong>of</strong> cables in separate cableways, minimum<br />
distance between high and low-current cables, types <strong>of</strong> cableways, etc.),<br />
electromagnetic coupling is at its maximum. Under these conditions, electronic<br />
equipment is sensitive to EMC disturbances flowing in the affected cables.<br />
Use <strong>of</strong> busbar trunking systems such as Canalis or busbar ducts for high power<br />
ratings is strongly advised. The levels <strong>of</strong> radiated magnetic fields using these types <strong>of</strong><br />
trunking systems is 10 to 20 times lower than standard cables or conductors.<br />
The recommendations in the “Cable running” and “Wiring recommendations”<br />
sections should be taken into account.<br />
3.4 False floors<br />
The inclusion <strong>of</strong> the floors in the mesh contributes to equipotentiality <strong>of</strong> the area and<br />
consequently to the distribution and dilution <strong>of</strong> disturbing LF currents.<br />
The screening effect <strong>of</strong> a false floor is directly related to its equipotentiality. If the<br />
contact between the floor plates is poor (rubber antistatic joints, for example) or if<br />
the contact between the support brackets is faulty (pollution, corrosion, mildew, etc.<br />
or if there are no support brackets), it is necessary to add an equipotential mesh. In<br />
this case, it is sufficient to ensure effective <strong>electrical</strong> connections between the metal<br />
support columns. Small spring clips are available on the market to connect the metal<br />
columns to the equipotential mesh. Ideally, each column should be connected, but<br />
it is <strong>of</strong>ten sufficient to connect every other column in each direction. A mesh 1.5 to<br />
2 metres is size is suitable in most cases. The recommended cross-sectional area <strong>of</strong><br />
the copper is 10 mm2 or more. In general, a flat braid is used. To reduce the effects <strong>of</strong><br />
corrosion, it is advised to use tin-plated copper (see Fig. Q8).<br />
Perforated floor plates act like normal floor plates when they have a cellular steel<br />
structure.<br />
Preventive maintenance is required for the floor plates approximately every five years<br />
(depending on the type <strong>of</strong> floor plate and the environment, including humidity, dust<br />
and corrosion). Rubber or polymer antistatic joints must be maintained, similar to the<br />
bearing surfaces <strong>of</strong> the floor plates (cleaning with a suitable product).<br />
Spring clips<br />
Metal support columns<br />
Fig. Q8 : False floor implementation<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
u 10 mm 2<br />
False floor<br />
Q7<br />
© Schneider Electric - all rights reserved
© Schneider Electric - all rights reserved<br />
Q8<br />
Q - EMC guidelines<br />
3 Implementation<br />
3. Cable running<br />
Selection <strong>of</strong> materials and their shape depends on the following criteria:<br />
b Severity <strong>of</strong> the EM environment along cableways (proximity <strong>of</strong> sources <strong>of</strong><br />
conducted or radiated EM disturbances)<br />
b Authorised level <strong>of</strong> conducted and radiated emissions<br />
b Type <strong>of</strong> cables (shielded?, twisted?, optical fibre?)<br />
b EMI withstand capacity <strong>of</strong> the equipment connected to the wiring system<br />
b Other environmental constraints (chemical, mechanical, climatic, fire, etc.)<br />
b Future extensions planned for the wiring system<br />
Non-metal cableways are suitable in the following cases:<br />
b A continuous, low-level EM environment<br />
b A wiring system with a low emission level<br />
b Situations where metal cableways should be avoided (chemical environment)<br />
b Systems using optical fibres<br />
For metal cableways, it is the shape (flat, U-shape, tube, etc.) rather than the crosssectional<br />
area that determines the characteristic impedance. Closed shapes are<br />
better than open shapes because they reduce common-mode coupling. Cableways<br />
<strong>of</strong>ten have slots for cable straps. The smaller the better. The types <strong>of</strong> slots causing<br />
the fewest problems are those cut parallel and at some distance from the cables.<br />
Slots cut perpendicular to the cables are not recommended (see Fig. Q9).<br />
Fig. Q10 : Installation <strong>of</strong> different types <strong>of</strong> cables<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Mediocre OK Better<br />
Fig. Q9 : CEM performance <strong>of</strong> various types <strong>of</strong> metal cableways<br />
In certain cases, a poor cableway in EMI terms may be suitable if the<br />
EM environment is low, if shielded cables or optical fibres are employed, or separate<br />
cableways are used for the different types <strong>of</strong> cables (power, data processing, etc.).<br />
It is a good idea to reserve space inside the cableway for a given quantity <strong>of</strong><br />
additional cables. The height <strong>of</strong> the cables must be lower than the partitions <strong>of</strong> the<br />
cableway as shown below. Covers also improve the EMC performance <strong>of</strong> cableways.<br />
In U-shaped cableways, the magnetic field decreases in the two corners.<br />
That explains why deep cableways are preferable (see Fig. Q10).<br />
NO!<br />
YES!<br />
Area protected against external EM field<br />
Different types <strong>of</strong> cables (power and low-level connections) should not be installed in<br />
the same bundle or in the same cableway. Cableways should never be filled to more<br />
than half capacity.
Q - EMC guidelines<br />
3 Implementation<br />
It is recommended to electromagnetically separate groups from one another, either<br />
using shielding or by installing the cables in different cableways. The quality <strong>of</strong> the<br />
shielding determines the distance between groups. If there is no shielding, sufficient<br />
distances must be maintained (see Fig. Q11).<br />
The distance between power and control cables must be at least 5 times the radius<br />
<strong>of</strong> the larger power cable.<br />
Forbidden Correct Ideal<br />
Power cables<br />
Auxiliary circuits (relay contacts)<br />
Control (digital)<br />
Measurements (analogue)<br />
Note: All metal parts must be <strong>electrical</strong>ly interconnected<br />
Fig. Q11 : Recommendation to install groups <strong>of</strong> cables in metal cableways<br />
Metal building components can be used for EMC purposes. Steel beams (L, H, U<br />
or T shaped) <strong>of</strong>ten form an uninterrupted earthed structure with large transversal<br />
sections and surfaces with numerous intermediate earthing connections. Cables<br />
should if possible be run along such beams. Inside corners are better than the<br />
outside surfaces (see Fig. Q12).<br />
Fig. Q12 : Recommendation to install cables in steel beams<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Recommended<br />
Acceptable<br />
Not recommended<br />
Both ends <strong>of</strong> metal cableways must always be connected to local earth electrodes.<br />
For very long cableways, additional connections to the earthing system are<br />
recommended between connected devices. Where possible, the distance between<br />
these earthing connections should be irregular (for symmetrical wiring systems) to<br />
avoid resonance at identical frequencies. All connections to the earthing system<br />
should be short.<br />
Metal and non-metal cableways are available. Metal solutions <strong>of</strong>fer better<br />
EMC characteristics. A cableway (cable trays, conduits, cable brackets, etc.) must<br />
<strong>of</strong>fer a continuous, conducting metal structure from beginning to end.<br />
An aluminium cableway has a lower DC resistance than a steel cableway <strong>of</strong> the<br />
same size, but the transfer impedance (Zt) <strong>of</strong> steel drops at a lower frequency,<br />
particularly when the steel has a high relative permeability µ r. Care must be taken<br />
when different types <strong>of</strong> metal are used because direct <strong>electrical</strong> connection is not<br />
authorised in certain cases to avoid corrosion. That could be a disadvantage in terms<br />
<strong>of</strong> EMC.<br />
When devices connected to the wiring system using unshielded cables are not<br />
affected by low-frequency disturbances, the EMC <strong>of</strong> non-metal cableways can be<br />
improved by adding a parallel earthing conductor (PEC) inside the cableway. Both<br />
ends must be connected to the local earthing system. Connections should be made<br />
to a metal part with low impedance (e.g. a large metal panel <strong>of</strong> the device case).<br />
The PEC should be <strong>design</strong>ed to handle high fault and common-mode currents.<br />
Q9<br />
© Schneider Electric - all rights reserved
Q10<br />
© Schneider Electric - all rights reserved<br />
Q - EMC guidelines<br />
3 Implementation<br />
Implementation<br />
When a metal cableway is made up <strong>of</strong> a number <strong>of</strong> short sections, care is required to<br />
ensure continuity by correctly bonding the different parts. The parts should preferably<br />
be welded along all edges. Riveted, bolted or screwed connections are authorised as<br />
long as the contact surfaces conduct current (no paint or insulating coatings) and are<br />
protected against corrosion. Tightening torques must be observed to ensure correct<br />
pressure for the <strong>electrical</strong> contact between two parts.<br />
When a particular shape <strong>of</strong> cableway is selected, it should be used for the entire<br />
length. All interconnections must have a low impedance. A single wire connection<br />
between two parts <strong>of</strong> the cableway produces a high local impedance that cancels its<br />
EMC performance.<br />
Starting at a few MHz, a ten-centimetre connection between two parts <strong>of</strong> the cableway<br />
reduces the attenuation factor by more than a factor <strong>of</strong> ten (see Fig. Q13).<br />
Fig. Q13 : Metal cableways assembly<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
NO!<br />
NOT RECOMMENDED<br />
YES!<br />
Each time modifications or extensions are made, it is very important to make sure<br />
they are carried out according to EMC <strong>rules</strong> (e.g. never replace a metal cableway by<br />
a plastic version!).<br />
Covers for metal cableways must meet the same requirements as those applying to<br />
the cableways themselves. A cover should have a large number <strong>of</strong> contacts along the<br />
entire length. If that is not possible, it must be connected to the cableway at least at<br />
the two ends using short connections (e.g. braided or meshed connections).<br />
When cableways must be interrupted to pass through a wall (e.g. firewalls), lowimpedance<br />
connections must be used between the two parts (see Fig. Q14).<br />
Mediocre OK Better<br />
Fig. Q14 : Recommendation for metal cableways assembly to pass through a wall
Q - EMC guidelines<br />
Fig. Q15 : Implementation <strong>of</strong> shielded cables<br />
All bonding connections must be made to bare metal<br />
Not acceptable<br />
Bonding wire<br />
Poorly connected shielding = reduced effectiveness<br />
Correct<br />
Equipotential metal panel<br />
3 Implementation<br />
3. Implementation <strong>of</strong> shielded cables<br />
When the decision is made to use shielded cables, it is also necessary to determine<br />
how the shielding will be bonded (type <strong>of</strong> earthing, connector, cable entry, etc.),<br />
otherwise the benefits are considerably reduced. To be effective, the shielding should<br />
be bonded over 360°. Figure Q1 below show different ways <strong>of</strong> earthing the cable<br />
shielding.<br />
For computer equipment and digital links, the shielding should be connected at each<br />
end <strong>of</strong> the cable.<br />
Connection <strong>of</strong> the shielding is very important for EMC and the following points should<br />
be noted.<br />
If the shielded cable connects equipment located in the same equipotential bonding<br />
area, the shielding must be connected to the exposed conductive parts (ECP) at<br />
both ends. If the connected equipment is not in the same equipotential bonding area,<br />
there are a number <strong>of</strong> possibilities.<br />
b Connection <strong>of</strong> only one end to the ECPs is dangerous. If an insulation fault occurs,<br />
the voltage in the shielding can be fatal for an operator or destroy equipment. In<br />
addition, at high frequencies, the shielding is not effective.<br />
b Connection <strong>of</strong> both ends to the ECPs can be dangerous if an insulation fault<br />
occurs. A high current flows in the shielding and can damage it. To limit this problem,<br />
a parallel earthing conductor (PEC) must be run next to the shielded cable. The size<br />
<strong>of</strong> the PEC depends on the short-circuit current in the given part <strong>of</strong> the <strong>installation</strong>.<br />
It is clear that if the <strong>installation</strong> has a well meshed earthing network, this problem<br />
does not arise.<br />
Collar, clamp, etc.<br />
Acceptable<br />
Ideal<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Collar, clamp, etc.<br />
Bonding bar<br />
connected<br />
to the chassis<br />
Cable gland = circumferential contact to<br />
equipotential metal panel<br />
3.7 Communication networks<br />
Communication networks cover large distances and interconnect equipment<br />
installed in rooms that may have distribution systems with different system earthing<br />
arrangements. In addition, if the various sites are not equipotential, high transient<br />
currents and major differences in potential may occur between the various devices<br />
connected to the networks. As noted above, this is the case when insulation<br />
faults and lightning strikes occur. The dielectric withstand capacity (between live<br />
conductors and exposed conductive parts) <strong>of</strong> communication cards installed in<br />
PCs or PLCs generally does not exceed 500 V. At best, the withstand capacity can<br />
reach 1.5 kV. In meshed <strong>installation</strong>s with the TN-S system and relatively small<br />
communication networks, this level <strong>of</strong> withstand capacity is acceptable. In all cases,<br />
however, protection against lightning strikes (common and differential modes) is<br />
recommended.<br />
Q11<br />
© Schneider Electric - all rights reserved
Q12<br />
© Schneider Electric - all rights reserved<br />
Q - EMC guidelines<br />
i<br />
VL1<br />
V arrestor<br />
VL2<br />
i<br />
Earthing bar<br />
Protected<br />
device<br />
Fig. Q16 : The protected device must be connected to the surge-arrestor terminals<br />
Earthing bar<br />
SA<br />
Isc<br />
prot<br />
SA<br />
3 Implementation<br />
The type <strong>of</strong> communication cable employed is an important parameter. It must<br />
be suited to the type <strong>of</strong> transmission. To create a reliable communication link, the<br />
following parameters must be taken into account:<br />
b Characteristic impedance<br />
b Twisted pairs or other arrangement<br />
b Resistance and capacitance per unit length<br />
b Signal attenutation per unit length<br />
b The type(s) <strong>of</strong> shielding used<br />
In addition, it is important to use symmetrical (differential) transmission links because<br />
they <strong>of</strong>fer higher performance in terms <strong>of</strong> EMC.<br />
In environments with severe EM conditions, however, or for wide communication<br />
networks between <strong>installation</strong>s that are not or are only slightly equipotential, in<br />
conjunction with IT, TT or TN-C systems, it is highly recommended to use optical<br />
fibre links.<br />
For safety reasons, the optical fibre must not have metal parts (risk <strong>of</strong> electric shock<br />
if the fibre links two areas with different potentials).<br />
3.8 Implementation <strong>of</strong> surge arrestors<br />
The wiring <strong>of</strong> surge arrestors is as important as the selection <strong>of</strong> the surge arrestor<br />
itself. Figures Q1 and Q17a below shows that the connection cables <strong>of</strong> the surge<br />
arrestor and its disconnection circuit-breaker must not exceed 50 centimetres to<br />
ensure effective protection.<br />
Common mode<br />
impedance<br />
1 m <strong>of</strong> cable = 1 µH<br />
L1 = 0.5 m = 0.5 µH<br />
L2 = 1.5 m = 1.5 µH<br />
di lightning = 10 kA<br />
dt = 10 µs<br />
V arrestor = 1,200 V<br />
Protected<br />
outgoers<br />
Common-mode impedance length y 50 cm<br />
Surge Arrestor<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Device withstand in common mode: 2,000 V<br />
V PROT = VL1 + VL2 + V arrestor<br />
where<br />
di<br />
10 kA<br />
VL1 = L1 = 0.5 µH x = 500 V<br />
dt<br />
10 µs<br />
di<br />
10 kA<br />
VL2 = L2 = 1.5 µH x = 1,500 V<br />
dt<br />
10 µs<br />
V PROT = 500 V + 1,500 V + 1,200 V = 3,200 V !<br />
L1 + L2 must therefore not exceed 0.5 m<br />
Protected<br />
outgoers<br />
Earthing bar<br />
Fig. Q17a : Examples <strong>of</strong> assemblies combining surge arrestors (SA) and disconnection circuit-breakers to reduce the common-mode impedances and the area <strong>of</strong><br />
upstream-downstream loops<br />
Isc<br />
prot<br />
SA
Q - EMC guidelines<br />
Fig. Q17b : The protected device must be connected to the surge-arrestor terminals<br />
3 Implementation<br />
3.9 Cabinet cabling (Fig. Q18)<br />
Each cabinet must be equipped with an earthing bar or a ground reference metal<br />
sheet. All shielded cables and external protection circuits must be connected to this<br />
point. Anyone <strong>of</strong> the cabinet metal sheets or the DIN rail can be used as the ground<br />
reference.<br />
Plastic cabinets are not recommended. In this case, the DIN rail must be used as<br />
ground reference.<br />
3.10 Standards<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Potential<br />
Reference Plate<br />
It is absolutely essential to specify the standards and recommendations that must be<br />
taken into account for <strong>installation</strong>s.<br />
Below are several documents that may be used:<br />
b EN 50174-1 Information technology - Cabling <strong>installation</strong>. Part 1:<br />
Specification and quality assurance<br />
b EN 50174-2 Information technology - Cabling <strong>installation</strong>. Part 2: Installation<br />
planning and practices inside buildings<br />
Q13<br />
© Schneider Electric - all rights reserved
Q14<br />
© Schneider Electric - all rights reserved<br />
Q - EMC guidelines<br />
4 Coupling mechanisms and<br />
counter-measures<br />
4.1 <strong>General</strong><br />
An EM interference phenomenon may be summed up in Figure Q18 below.<br />
Source Coupling<br />
Victim<br />
Origin <strong>of</strong><br />
emitted disturbances<br />
Example:<br />
Walkie-talkie TV set<br />
Fig. Q18 : EM interference phenomenon<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Means by which<br />
disturbances are<br />
transmitted<br />
Radiated waves<br />
Equipment likely<br />
to be disturbed<br />
The different sources <strong>of</strong> disturbances are:<br />
b Radio-frequency emissions<br />
v Wireless communication systems (radio, TV, CB, radio telephones, remote controls)<br />
v Radar<br />
b Electrical equipment<br />
v High-power industrial equipment (induction furnaces, welding machines, stator<br />
control systems)<br />
v Office equipment (computers and electronic circuits, photocopy machines, large<br />
monitors)<br />
v Discharge lamps (neon, fluorescent, flash, etc.)<br />
v Electromechanical components (relays, contactors, solenoids, current interruption<br />
devices)<br />
b Power systems<br />
v Power transmission and distribution systems<br />
v Electrical transportation systems<br />
b Lightning<br />
b Electrostatic discharges (ESD)<br />
b Electromagnetic nuclear pulses (EMNP)<br />
The potential victims are:<br />
b Radio and television receivers, radar, wireless communication systems<br />
b Analogue systems (sensors, measurement acquisition, amplifiers, monitors)<br />
b Digital systems (computers, computer communications, peripheral equipment)<br />
The different types <strong>of</strong> coupling are:<br />
b Common-mode impedance (galvanic) coupling<br />
b Capacitive coupling<br />
b Inductive coupling<br />
b Radiated coupling (cable to cable, field to cable, antenna to antenna)
Q - EMC guidelines<br />
4 Coupling mechanisms and<br />
counter-measures<br />
4.2 Common-mode impedance coupling<br />
Definition<br />
Two or more devices are interconnected by the power supply and communication<br />
cables (see Fig. Q19). When external currents (lightning, fault currents, disturbances)<br />
flow via these common-mode impedances, an undesirable voltage appears between<br />
points A and B which are supposed to be equipotential. This stray voltage can<br />
disturb low-level or fast electronic circuits.<br />
All cables, including the protective conductors, have an impedance, particularly at<br />
high frequencies.<br />
Stray<br />
overvoltage<br />
Z1<br />
Fig. Q19 : Definition <strong>of</strong> common-mode impedance coupling<br />
Examples (see Fig. Q20)<br />
Device 1<br />
b Devices linked by a common reference conductor (e.g. PEN, PE) affected by fast<br />
or intense (di/dt) current variations (fault current, lightning strike, short-circuit, load<br />
changes, chopping circuits, harmonic currents, power factor correction capacitor<br />
banks, etc.)<br />
b A common return path for a number <strong>of</strong> <strong>electrical</strong> sources<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Z sign.<br />
I2<br />
A B<br />
I1<br />
Z2<br />
Device 2<br />
ECPs ECPs<br />
Signal line<br />
The exposed conductive parts (ECP) <strong>of</strong> devices 1 and 2 are connected to a common<br />
earthing terminal via connections with impedances Z1 and Z2.<br />
The stray overvoltage flows to the earth via Z1. The potential <strong>of</strong> device 1 increases<br />
to Z1 I1. The difference in potential with device 2 (initial potential = 0) results in the<br />
appearance <strong>of</strong> current I2.<br />
I2<br />
Z1<br />
Z1 I1= ( Zsign + Z2)<br />
I2<br />
⇒ =<br />
I1<br />
Zsign + Z2<br />
( )<br />
I<br />
Current I2, present on the signal line, disturbs device 2.<br />
Device 1 Device 2<br />
Signal cable<br />
Disturbing<br />
current<br />
Difference in<br />
potential<br />
ZMC<br />
Disturbed<br />
cable<br />
Fig. Q20 : Example <strong>of</strong> common-mode impedance coupling<br />
Fault<br />
currents<br />
Lightning<br />
strike<br />
Q15<br />
© Schneider Electric - all rights reserved
Q16<br />
© Schneider Electric - all rights reserved<br />
Q - EMC guidelines<br />
U<br />
Vsource<br />
Vvictim<br />
Fig. Q22 : Typical result <strong>of</strong> capacitive coupling (capacitive<br />
cross-talk)<br />
t<br />
t<br />
4 Coupling mechanisms and<br />
counter-measures<br />
Counter-measures (see Fig. Q21)<br />
If they cannot be eliminated, common-mode impedances must at least be as low as<br />
possible. To reduce the effects <strong>of</strong> common-mode impedances, it is necessary to:<br />
b Reduce impedances:<br />
v Mesh the common references,<br />
v Use short cables or flat braids which, for equal sizes, have a lower impedance than<br />
round cables,<br />
v Install functional equipotential bonding between devices.<br />
b Reduce the level <strong>of</strong> the disturbing currents by adding common-mode filtering and<br />
differential-mode inductors<br />
Stray<br />
overvoltage<br />
Z1<br />
Device 1<br />
If the impedance <strong>of</strong> the parallel earthing conductor PEC (Z sup) is very low<br />
compared to Z sign, most <strong>of</strong> the disturbing current flows via the PEC, i.e. not<br />
via the signal line as in the previous case.<br />
The difference in potential between devices 1 and 2 becomes very low and the<br />
disturbance acceptable.<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
I1<br />
Z sign.<br />
I2<br />
Z sup.<br />
Fig. Q21 : Counter-measures <strong>of</strong> common-mode impedance coupling<br />
4.3 Capacitive coupling<br />
Definition<br />
Z2<br />
Device 2<br />
The level <strong>of</strong> disturbance depends on the voltage variations (dv/dt) and the value <strong>of</strong><br />
the coupling capacitance between the disturber and the victim.<br />
Capacitive coupling increases with:<br />
b The frequency<br />
b The proximity <strong>of</strong> the disturber to the victim and the length <strong>of</strong> the parallel cables<br />
b The height <strong>of</strong> the cables with respect to a ground referencing plane<br />
b The input impedance <strong>of</strong> the victim circuit (circuits with a high input impedance are<br />
more vulnerable)<br />
b The insulation <strong>of</strong> the victim cable (ε r <strong>of</strong> the cable insulation), particularly for tightly<br />
coupled pairs<br />
Figure Q22 shows the results <strong>of</strong> capacitive coupling (cross-talk) between two cables.<br />
Examples (see Fig. Q23 opposite page)<br />
b Nearby cables subjected to rapid voltage variations (dv/dt)<br />
b Start-up <strong>of</strong> fluorescent lamps<br />
b High-voltage switch-mode power supplies (photocopy machines, etc.)<br />
b Coupling capacitance between the primary and secondary windings <strong>of</strong><br />
transformers<br />
b Cross-talk between cables<br />
PEC
Q - EMC guidelines<br />
Source<br />
Metal shielding<br />
Fig. Q24 : Cable shielding with perforations reduces capacitive<br />
coupling<br />
C<br />
Victim<br />
4 Coupling mechanisms and<br />
counter-measures<br />
Differential mode Common mode<br />
Vs<br />
DM<br />
Source<br />
Counter-measures (see Fig. Q24)<br />
b Limit the length <strong>of</strong> parallel runs <strong>of</strong> disturbers and victims to the strict minimum<br />
b Increase the distance between the disturber and the victim<br />
b For two-wire connections, run the two wires as close together as possible<br />
b Position a PEC bonded at both ends and between the disturber and the victim<br />
b Use two or four-wire cables rather than individual conductors<br />
b Use symmetrical transmission systems on correctly implemented, symmetrical<br />
wiring systems<br />
b Shield the disturbing cables, the victim cables or both (the shielding must be<br />
bonded)<br />
b Reduce the dv/dt <strong>of</strong> the disturber by increasing the signal rise time where possible<br />
4.4 Inductive coupling<br />
Definition<br />
Iv<br />
DM<br />
Victim<br />
Vs DM: Source <strong>of</strong> the disturbing voltage (differential mode)<br />
Iv DM: Disturbing current on victim side (differential mode)<br />
Vs CM: Source <strong>of</strong> the disturbing voltage (common mode)<br />
Iv CM: Disturbing current on victim side (common mode)<br />
Fig. Q23 : Example <strong>of</strong> capacitive coupling<br />
The disturber and the victim are coupled by a magnetic field. The level <strong>of</strong> disturbance<br />
depends on the current variations (di/dt) and the mutual coupling inductance.<br />
Inductive coupling increases with:<br />
b The frequency<br />
b The proximity <strong>of</strong> the disturber to the victim and the length <strong>of</strong> the parallel cables,<br />
b The height <strong>of</strong> the cables with respect to a ground referencing plane,<br />
b The load impedance <strong>of</strong> the disturbing circuit.<br />
Examples (see Fig. Q25 next page)<br />
b Nearby cables subjected to rapid current variations (di/dt)<br />
b Short-circuits<br />
b Fault currents<br />
b Lightning strikes<br />
b Stator control systems<br />
b Welding machines<br />
b Inductors<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Vs<br />
CM<br />
Source Victim<br />
Iv<br />
CM<br />
Q17<br />
© Schneider Electric - all rights reserved
Q18<br />
© Schneider Electric - all rights reserved<br />
Q - EMC guidelines<br />
4 Coupling mechanisms and<br />
counter-measures<br />
Counter-measures<br />
b Limit the length <strong>of</strong> parallel runs <strong>of</strong> disturbers and victims to the strict minimum<br />
b Increase the distance between the disturber and the victim<br />
b For two-wire connections, run the two wires as close together as possible<br />
b Use multi-core or touching single-core cables, preferably in a triangular layout<br />
b Position a PEC bonded at both ends and between the disturber and the victim<br />
b Use symmetrical transmission systems on correctly implemented, symmetrical<br />
wiring systems<br />
b Shield the disturbing cables, the victim cables or both (the shielding must be<br />
bonded)<br />
b Reduce the dv/dt <strong>of</strong> the disturber by increasing the signal rise time where possible<br />
(series-connected resistors or PTC resistors on the disturbing cable, ferrite rings on<br />
the disturbing and/or victim cable)<br />
4.5 Radiated coupling<br />
Definition<br />
i<br />
Disturbing<br />
cable<br />
H<br />
Victim pair<br />
Victim loop<br />
Differential mode Common mode<br />
Fig. Q25 : Example <strong>of</strong> inductive coupling<br />
The disturber and the victim are coupled by a medium (e.g. air). The level <strong>of</strong><br />
disturbance depends on the power <strong>of</strong> the radiating source and the effectiveness<br />
<strong>of</strong> the emitting and receiving antenna. An electromagnetic field comprises both an<br />
<strong>electrical</strong> field and a magnetic field. The two fields are correlated. It is possible to<br />
analyse separately the <strong>electrical</strong> and magnetic components.<br />
The <strong>electrical</strong> field (E field) and the magnetic field (H field) are coupled in wiring<br />
systems via the wires and loops (see Fig. Q26).<br />
E field<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
i<br />
i<br />
Disturbing<br />
cable<br />
Field-to-cable coupling Field-to-loop coupling<br />
Fig. Q26 : Definition <strong>of</strong> radiated coupling<br />
H<br />
H field<br />
V<br />
Victim loop
Q - EMC guidelines<br />
4 Coupling mechanisms and<br />
counter-measures<br />
When a cable is subjected to a variable <strong>electrical</strong> field, a current is generated in the<br />
cable. This phenomenon is called field-to-cable coupling.<br />
Similarly, when a variable magnetic field flows through a loop, it creates a counter<br />
electromotive force that produces a voltage between the two ends <strong>of</strong> the loop. This<br />
phenomenon is called field-to-loop coupling.<br />
Examples (see Fig. Q27)<br />
b Radio-transmission equipment (walkie-talkies, radio and TV transmitters, mobile<br />
services)<br />
b Radar<br />
b Automobile ignition systems<br />
b Arc-welding machines<br />
b Induction furnaces<br />
b Power switching systems<br />
b Electrostatic discharges (ESD)<br />
b Lighting<br />
Device<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
h<br />
Ground reference plane<br />
i<br />
EM field<br />
Signal<br />
cable<br />
Device 1 Device 2<br />
Example <strong>of</strong> field-to-cable coupling Example <strong>of</strong> field-to-loop coupling<br />
Fig. Q27 : Examples <strong>of</strong> radiated coupling<br />
Counter-measures<br />
E field<br />
h<br />
Area <strong>of</strong> the<br />
earth loop<br />
To minimise the effects <strong>of</strong> radiated coupling, the measures below are required.<br />
For field-to-cable coupling<br />
b Reduce the antenna effect <strong>of</strong> the victim by reducing the height (h) <strong>of</strong> the cable with<br />
respect to the ground referencing plane<br />
b Place the cable in an uninterrupted, bonded metal cableway (tube, trunking, cable<br />
tray)<br />
b Use shielded cables that are correctly installed and bonded<br />
b Add PECs<br />
b Place filters or ferrite rings on the victim cable<br />
For field-to-loop coupling<br />
b Reduce the surface <strong>of</strong> the victim loop by reducing the height (h) and the length<br />
<strong>of</strong> the cable. Use the solutions for field-to-cable coupling. Use the Faraday cage<br />
principle.<br />
Radiated coupling can be eliminated using the Faraday cage principle. A possible<br />
solution is a shielded cable with both ends <strong>of</strong> the shielding connected to the metal<br />
case <strong>of</strong> the device. The exposed conductive parts must be bonded to enhance<br />
effectiveness at high frequencies.<br />
Radiated coupling decreases with the distance and when symmetrical transmission<br />
links are used.<br />
Q19<br />
© Schneider Electric - all rights reserved
Q20<br />
© Schneider Electric - all rights reserved<br />
Q - EMC guidelines<br />
Unshielded cables <strong>of</strong><br />
different groups<br />
NO!<br />
h<br />
YES!<br />
Risk <strong>of</strong> cross-talk in common mode if e < 3 h<br />
Sensitive<br />
cable<br />
Disturbing<br />
cable<br />
e<br />
Shielded cables <strong>of</strong><br />
different groups<br />
Sensitive<br />
cable<br />
Disturbing<br />
cable<br />
30 cm u 1 m<br />
NO! YES!<br />
Ground<br />
reference<br />
plane<br />
Cross incompatible<br />
cables at right angles<br />
Fig. Q29 : Wiring recommendations for cables carrying<br />
different types <strong>of</strong> signals<br />
NO! YES!<br />
Standard cable<br />
Poorly implemented<br />
ribbon cable<br />
Digital connection<br />
Analogue pair<br />
Bonding wires<br />
Fig. Q30 : Use <strong>of</strong> cables and ribbon cable<br />
Two distinct pairs<br />
Correctly implemented<br />
ribbon cable<br />
5 Wiring recommendations<br />
5.1 Signal classes (see Fig. Q28)<br />
1 - Power connections<br />
(supply + PE)<br />
4 - Analogue link<br />
(sensor)<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
Device<br />
Fig. Q28 : Internal signals can be grouped in four classes<br />
2 - Relay<br />
connections<br />
3 - Digital link<br />
(bus)<br />
Four classes <strong>of</strong> internal signals are:<br />
b Class 1<br />
Mains power lines, power circuits with a high di/dt, switch-mode converters, powerregulation<br />
control devices.<br />
This class is not very sensitive, but disturbs the other classes (particularly in<br />
common mode).<br />
b Class 2<br />
Relay contacts.<br />
This class is not very sensitive, but disturbs the other classes (switching, arcs when<br />
contacts open).<br />
b Class 3<br />
Digital circuits (HF switching).<br />
This class is sensitive to pulses, but also disturbs the following class.<br />
b Class 4<br />
Analogue input/output circuits (low-level measurements, active sensor supply<br />
circuits). This class is sensitive.<br />
It is a good idea to use conductors with a specific colour for each class to<br />
facilitate identification and separate the classes. This is useful during <strong>design</strong> and<br />
troubleshooting.<br />
5.2 Wiring recommendations<br />
Cables carrying different types <strong>of</strong> signals must be physically separated<br />
(see Fig. Q29 above)<br />
Disturbing cables (classes 1 and 2) must be placed at some distance from the<br />
sensitive cables (classes 3 and 4) (see Fig. Q29 and Fig. Q30)<br />
In general, a 10 cm separation between cables laid flat on sheet metal is sufficient<br />
(for both common and differential modes). If there is enough space, a distance <strong>of</strong><br />
30 cm is preferable. If cables must be crossed, this should be done at right angles to<br />
avoid cross-talk (even if they touch). There are no distance requirements if the cables<br />
are separated by a metal partition that is equipotential with respect to the ECPs.<br />
However, the height <strong>of</strong> the partition must be greater than the diameter <strong>of</strong> the cables.
Q - EMC guidelines<br />
Power +<br />
analogue<br />
NO! YES!<br />
Digital +<br />
relay contacts<br />
Power connections<br />
Relay I/O connections<br />
Power +<br />
relay contacts<br />
Shielding<br />
Fig. Q31 : Incompatible signals = different cables<br />
NO! YES!<br />
Digital connections<br />
Analogue connections<br />
Digital +<br />
analogue<br />
Digital connections<br />
Analogue connections<br />
Fig. Q33 : Segregation applies to connectors as well!<br />
5 Wiring recommendations<br />
A cable should carry the signals <strong>of</strong> a single group (see Fig. Q31)<br />
If it is necessary to use a cable to carry the signals <strong>of</strong> different groups, internal<br />
shielding is necessary to limit cross-talk (differential mode). The shielding, preferably<br />
braided, must be bonded at each end for groups 1, 2 and 3.<br />
It is advised to overshield disturbing and sensitive cables (see Fig. Q32)<br />
The overshielding acts as a HF protection (common and differential modes) if it<br />
is bonded at each end using a circumferential connector, a collar or a clampere<br />
However, a simple bonding wire is not sufficient.<br />
Electronic<br />
control<br />
device<br />
Electronic<br />
control<br />
device<br />
Shielded pair<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
NO!<br />
Unshielded cable for stator control<br />
YES!<br />
Shielded pair + overshielding<br />
Shielded cable for stator control<br />
Electromechanical<br />
device<br />
Electromechanical<br />
device<br />
Fig. Q32 : Shielding and overshielding for disturbing and/or sensitive cables<br />
Sensor<br />
Bonded using a clamp<br />
Sensor<br />
Avoid using a single connector for different groups (see Fig. Q33)<br />
Except where necessary for groups 1 and 2 (differential mode). If a single connector<br />
is used for both analogue and digital signals, the two groups must be separated by at<br />
least one set <strong>of</strong> contacts connected to 0 V used as a barrier.<br />
All free conductors (reserve) must always be bonded at each end<br />
(see Fig. Q34)<br />
For group 4, these connections are not advised for lines with very low voltage<br />
and frequency levels (risk <strong>of</strong> creating signal noise, by magnetic induction, at the<br />
transmission frequencies).<br />
Wires not<br />
equipotentially<br />
bonded<br />
NO! YES!<br />
Electronic<br />
system<br />
Equipotential sheet metal panel<br />
Fig. Q34 : Free wires must be equipotentially bonded<br />
Electronic<br />
system<br />
Equipotential sheet metal panel<br />
Q21<br />
© Schneider Electric - all rights reserved
Q22<br />
© Schneider Electric - all rights reserved<br />
Q - EMC guidelines<br />
NO! YES!<br />
Power or disturbing cables<br />
Relay cables<br />
Measurement or sensitive cables<br />
Fig. Q37 : Cable distribution in cable trays<br />
Metal tray<br />
5 Wiring recommendations<br />
The two conductors must be installed as close together as possible<br />
(see Fig. Q35)<br />
This is particularly important for low-level sensors. Even for relay signals with a<br />
common, the active conductors should be accompanied by at least one common<br />
conductor per bundle. For analogue and digital signals, twisted pairs are a minimum<br />
requirement. A twisted pair (differential mode) guarantees that the two wires remain<br />
together along their entire length.<br />
PCB with<br />
relay contact<br />
I/Os<br />
- +<br />
Power supply<br />
Schneider Electric - Electrical <strong>installation</strong> guide 2008<br />
NO! YES!<br />
Area <strong>of</strong><br />
loop too large<br />
PCB with<br />
relay contact<br />
I/Os<br />
Fig. Q35 : The two wires <strong>of</strong> a pair must always be run close together<br />
- +<br />
Power supply<br />
Group-1 cables do not need to be shielded if they are filtered<br />
But they should be made <strong>of</strong> twisted pairs to ensure compliance with the previous<br />
section.<br />
Cables must always be positioned along their entire length against the bonded<br />
metal parts <strong>of</strong> devices (see Fig. Q36)<br />
For example: Covers, metal trunking, structure, etc. In order to take advantage <strong>of</strong> the<br />
dependable, inexpensive and significant reduction effect (common mode) and anticross-talk<br />
effect (differential mode).<br />
Power<br />
supply<br />
NO! YES!<br />
Chassis 1<br />
Chassis 2<br />
Chassis 3<br />
I/O interface<br />
Power<br />
supply<br />
Chassis 1<br />
Chassis 2<br />
Chassis 3<br />
I/O interface<br />
All metal parts (frame, structure, enclosures, etc.) are equipotential<br />
Fig. Q36 : Run wires along their entire length against the bonded metal parts<br />
The use <strong>of</strong> correctly bonded metal trunking considerably improves<br />
internal EMC (see Fig. Q37)
Schneider Electric Industries SAS<br />
Head Office<br />
89, bd Franklin Roosevelt<br />
92506 Rueil-Malmaison Cedex<br />
FRANCE<br />
EIGED306001EN<br />
ART. 822690<br />
As standards, specifications and <strong>design</strong>s change from time, please ask for confirmation <strong>of</strong> the<br />
information given in this publication.<br />
ISBN 978-2-9531643-0-5<br />
This document has been<br />
printed on ecological paper<br />
9 782953 164305<br />
03/2008